I teach HS Ceramics and have 2 wheels (granted I see them every day). I keep track and have 2 students work on the wheel for each project that we do. The rest of the class is working on the given project. Since you only see your kids twice a week, perhaps you could use the wheel as a reward - develop some type of point system for kids who are always on task, have a good attitude, etc. If you are going to try to get more wheels, send home tax credit forms for parents and purchase wheels that way. Good luck!!