One of the quickest ways to make any space look more grown-up is to frame your posters instead of tacking or taping them straight onto the wall. If your school doesn't allow you to put nails in the wall, here are a couple workarounds:
1. Use adhesive picture-hanging strips to hang frames. I've used these for years to hang everything from vintage postcards to gigantic 25-lb frames. They're easier to use than nails, they hold securely, and you can remove them without damaging the wall.