I take part in a homeschool co-op that is held at a local church once a week, and I taught two art classes last year. The first was a single semester of drawing and pyrography (woodburning). My class was mixed genders and ages, 12 students ranging from age 8 to age 14. That class lasted 90 minutes. It was lots of fun and most of the students turned out some really nice work. Those who tried, anyway! I taught a technique on large paper or a white board in front of the class and then walked around among the students helping.
I also led a general art class for younger children for the whole school year. Started with about 6 students, but it ballooned into 16 by the end of the year, ranging from ages 3 to 9. That was crazy. Thankfully it only lasted 45 minutes! I taught color mixing and other color lessons and techniques, drawing from memory, drawing from a model, shading and shadows, the difference between realism and other forms, and such as that. But toward the last few weeks it was so large and so wild with so many children that I had to abandon the art lessons and began teaching about animals using some materials I had on hand from a wildlife class I was doing earlier in the day. Painting had become out of the question. I was not satisfied at all with how that class ended.
Some things I learned:
The age range for both classes was a bit too wide. The abilities and attention spans varied so much! I'm usually all for mixed ages, but it caused problems because the younger ones wanted to tell me all about their aunt's new puppy or how they saw a turtle cross the road last week, while meanwhile the older ones were drifting because they were waiting for the youngers to get back on task. If I'd had a choice, I would have preferred to have the 8-10 year olds and 11-14 year olds in separate groups for the drawing/woodburning class. And for the general art class it would have been great to have 5 and under and 6 and up. But there was no recourse for it, too few teachers.
Also, the interest level in art also varied widely... some children only took the class because there was nothing else offered in that time slot. Big difference in attitude between those who
wanted to be there and those who
had to be there. Some kids didn't even like art, or anything about art, yet here they were in an art class, sulking and doodling Star Wars Lego figures when we were trying to work on learning about how to draw textured fur. I would've much rather only had children who wanted to learn, because the sulkers and dissenters got everyone else off target so that even the ones who cared were negatively influenced. This was a tough situation that was never fully remedied, but could have been prevented by having other class choices for those who didn't care one bit for art.
We had a spacious room with three long tables set in a horseshoe shape for the older kids' class, and I would present from the open end of the horseshoe. Having ample space between children is paramount, to reduce talking for one thing but also so they won't be constantly comparing each other's work. And we were working with 900 degree tools for part of the time... space was definitely needed!

A woodburning pen is NOT a light sabre nor should it be used as such.

This room was a youth Sunday School room in the church with a storage closet in which to keep the supplies, cords, etc. It would have been hard to have such a large class in a regular home, although I suppose some folks have big houses.
For the younger children, space became a big issue as the class grew. We had to keep moving the class, and it was bad for continuity. The group was just plain old too large and would have been better served if it had been divided. But there weren't enough teachers... so many of the parents of younger children either didn't feel ready to teach in the co-op or were tending nurslings who were in bad need of a nap at that time of day. It would have been a much better year if there had been ample help and space. Besides that, 1:45 to 2:30 in the afternoon is a horrendous time of day for most little kids who have already been at co-op since 10. I would have much rather switched the classes to have younger kids in the morning and the older ones in the afternoon.
As for supplies, for the older class, we had a class fee that went toward supplies. I had $30 per student for the woodburning class, and this covered a 9x13 sketchbook, a set of drawing pencils in varying hardnesses, good white eraser and kneaded eraser, tortillions, sharpeners, pencil case, woodburning tool and tips and thrift store ceramic ashtrays to hold their woodburners. My dad donated the boards. I picked up the supplies myself, finding a good sale at Hobby Lobby. I wanted all the students to have the same exact supplies so I could teach them the same methods. It worked well and they got some quality things to keep in the end. During the semester, the students could choose to take their drawing tools home with them and bring them back each week... or if they did not trust themselves to bring the things back, they could store them in the church closet in the room we were using. About half took stuff home and half stored them. The ones who chose to take their stuff home never once forgot to bring them back, but if those others who opted out of this had taken them home.... we might have never seen those supplies again! We stored all the woodburners and ashtrays at church until the last class.
For the youngers, I had a class fee of $10 each that went toward a collection of paints, brushes, paper pads, etc. Students did not keep their own supplies. I noticed that many of them were wasteful of the supplies because they weren't their own. Most of the class never paid their fee as they straggled in toward the end of the year. It was all very haphazardly handled toward the end. Can you tell I just wasn't happy with that class?
To sum up and help actually answer your questions:
1. Age groups?
Try to keep them tight... at least like 5 and under, 6 to 10, 11 and up. But interest level is just as important. Make sure everyone who participates WANTS to be there. Have plenty of helpers for younger children, even if it is just to keep them on task or to prevent them from monopolizing your teaching time with off-topic talking. An interested, eager 6 year old who wants to learn could work in a class with a 12 year old if they are all kept on task. A child of any age who is not eager or interested may not be ready for a dedicated art class, because they will likely be a detriment to those who really want to learn and will have a huge negative impact on the teachers and the other students... especially if they are vocal about their dislike of art.
2. Supply purchase and storage?
Make a class fee and stick to it. Use the money to get everyone the same materials, maybe buying online in bulk if needed. Art kits which claim to have "everything you need" often don't really have everything you need, and it may be cheaper and better to get the items individually to suit your students' needs. Try as much as possible to get supplies which they will be able to keep at the the end. Children will take much better care of their own things than if they are just using something from a collective pool. Be sure to get a case or something and lable it to keep everyone's items separate, and if the children or their parents aren't trustworthy to bring the items back each week, store them at the location of the class. For simplicity's sake you may just want to store everyone's at the class, depending on what materials you have. If you are rotating from one location to another, make someone be in charge of bringing the items each week, perhaps using a storage container with rollers.
3. Places to hold the class?
This is a tough one. I would not opt for rotating between homes if possible, as much of the class time will be used in the children getting settled into a new environment. It is best to have a place that you could leave set up, but that may be hard to find. We couldn't leave our stuff set up because the church used those rooms as classrooms, so weekly clean-up was hard. Check with the families involved... someone may have a basement or workshop that is not being used for anything else. Barring that, do your best to make it consistent. And make sure it is as spacious as possible so the children aren't elbow to elbow. You'll need room to get beside them to assist. Make sure there is good light in the room, too... very important! Ventilation might be a consideration if you're painting. Or if you're doing pyrography, but hardly anybody ever does that. Except weirdos like me and those 12 wacky homeschooled kids I taught!

(If anybody wants to know about woodburning, just PM me and I will go on and on about it!)
Hope this helps a little. I learned some lessons the hard way.
I'm not doing art classes this year... have decided on teaching Apologia's Zoology 1 instead. With a class limit of 10! I learned my lesson. I'm also only teaching one class per week instead of two. Phew!
Some parents have shown interest in me leading an "advanced drawing" class, but it would have to be VERY small (5 or less) and I think it would have to be done by application or something.... just to make sure they really wanted to be there and didn't just take it because their friend is in there. I would love love love to teach a handful of students who REALLY wanted to learn. What a joy that would be!
I'm Shawna...
... a forgiven child of God since 1994 (age 16)
... happily wed to William since 1996
... mother of our long-awaited Gail (3/15/2006)
... missing 6 little ones (4 miscarriages, 2 ectopics)
... starting Rome to the Reformation this fall!