Rubber bands are banned at some schools to prevent students from shooting projectiles, known as “wasps” at each other. Buffalo’s school district warned:
If students are in possession of rubber bands for any reason they will be subject to consequences that may include out of school suspension. When rubber bands are required for classroom use, they will be provided and collected.
Another link, provided by Reginlief, makes the case for banning condoms in school: They make dangerous slingshots.