The velcro actually works really well, but is extremely difficult to get off. I only use it for Boomwhackers because I've been given permission by the custodians, and because I know that I'll never want to move them.
Enter Scotch Tough Duct Tape Transparent.
*It tears easily (no scissors necessary)
*It's transparent (no silver-gray residue left behind)
*It works (seriously)
I will warn you, not all walls are created equal. So before you spend a lot of money thinking you've found the cure for all of your sticking needs, see if anyone at your school has this tape and is willing to lend you some. If so, put at least four pieces on the back of a poster, and see if it holds for an entire week. If so, your walls and my walls are long-lost relatives. If not, I'm sorry. You have inherited the most difficult of walls.
If you decide to try this out, please leave a message in comments and let me know how it works for you.