This is great. I picked up so many ideas for next year - thanks.
One item that some of our classrooms did this year was a floor cloth. One of our moms is an artist and prepared the canvas (said it's not that hard to do) and they we sectioned off the rectangle into 24 different squares. We had volunteers come in and pull the kids out of class or recess to work on their square. The down side to this is you have to start really early because you have to take one student at time and wait for the acrylic paint to dry. It was worth it tho - the cloths we extraordinary and brought in big bucks, after bidding wars ; )
You could have the students collaborate on a few paintings. Pick something that you know will sell (based on a nice photo, graphic design project, classic art piece, modern art piece etc) and have them re-create it on canvas. With some assistance from the art teacher, they will turn out like the type of artwork you would buy at a store. These art pieces could help with the decor of the room as well.
I've done a couple classroom auction projects one website kidsartproject.com is amazing because you can have the kids draw what theme you want them to do then send it out to this company and she will pull it together and put it on thick gallery quality poster, plus we went through different color backgrounds and letter fonts. I did 2 posters with this company I copied one from her website, the Kindergarten does Kandinsky one which is amazing and then I was inspired by the "Hearts" poster but tweaked it to go with our religious theme and had our kindergarteners paint crosses instead. Another project I did was with our graduating 8th graders and it was with a nice console table that was donated by a local furniture store. It had a tile top which I popped out, dremmeled out the internal cross bars and laid a thin concrete board in it, I had pictures of the 8th graders taken and sent it off to a tile company called Silver Graphics Studios to have those pictures transferred to 4x4's and then placed those on the table, we had a little bit of room left so in the middle I had the kids sign their autographs had the tile company put that on larger tiles and an artist draw up an artsy cross which was also transferred to a tile then I went to a mosaic shop to get our school colors in tile chips to fill in the spaces then I grouted and sealed it, a bit time consuming but fun to do!!!
I made for an auction item for a 2nd grade class were I Spy end tables. Donated were two tables that were painted to look like a woods theme (sky, grass, trees) The center of the tables were framed in so that the epoxy would not leak out over the whole table. Pictures were taken of the kids faces, they got to pick their "bug" or "animal" and their faces were merged onto the critters, printed out and cutout and then placed in the square amongst fake flowers, sand, sticks, rocks, broken glass to look like a lake with a fish and beavers, frogs, birds, butterflies, caterpillars, some were flowers and fruit. Epoxy was poured over the project as directed on the can, took two days and poured two times. Dried clear, no bubbles, turned out great. Went for $550