I have a sticker in my back window for my university

Supposedly it can be removed by using a hair dryer according to the directions.
When I got the car I tried removing all of the state park stickers my brother put in the front window. A razor took of most of it, but a lot of the sun-baked glue stayed. It took a lot of elbow grease and ammonia-based cleaner to get it off, and in the right light I can still see shadows of the stickers