If the locker you want to decorate is kept locked, get the key or combination in advance so you can surprise the locker's owner on her birthday. Check with your school's handbook to see if there are any rules regarding decorating lockers and make sure you don't damage school or personal property.
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Outside the Locker
Make a vertical birthday banner to cover the front of the door. Have one made at a print shop or make one out of several pieces of printing paper taped together. Use colored paper to make birthday signs you can tape to the outside of the locker. On the sign, add pictures of the person celebrating past birthdays.
Up, Up and Not Away
Fill several latex or Mylar balloons with helium. Add a ribbon to each balloon then tie the ends of the ribbons on the locker's hook so they don't float away. Once they are tied in, stuff the balloons inside the locker and close the door. When the birthday person opens the locker, all the balloons will float out.
Buy small candies such as Hershey's Kisses or miniature chocolate bars. Roll small pieces of transparent tape onto the back of each piece and stick it to the inside of the locker door. Depending on how much chocolate you bought and how much time you have, you can cover the inside of the door and some of the walls.
Sprinkle metallic birthday confetti into the pages of her notebooks and binders inside the locker. She'll find all the little "happy birthday" messages throughout the day as she goes from class to class. Keep in mind that this idea can be a little messy.