OK my friend, this is what you do.
first you buy one of these:
Flexible Magnetic Sheet 4x12 inch for magnetizing bumper stickers, material film | eBay
you put the sticker on it, cut it to the size of the sticker, and that's it.
you get to school, you stick it to the back of the car. you leave school, you put it in the glove compartment (that you don't have ... but you get the point).