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Halloween decorations - outside the classroom door. What fun. I had two really unpredictable students in my ED class. Gave them a bunch of materials, not much in the way of directions and a little time. Had the hands down best display ever. Autonomy rocks. I can see giving students a picture, some old clothes and shoes and seeing what they come up with.
Bats -- I've done this for the past two years. The bats can be stored flat to be used for the following year. It is definitely creepy and everyone loves them. You can also do this around your door outside or on railings, etc.