I'm not sure about school but at this place I volunteer at, I do like this girl however I thought no one would be able to tell, but a coworker of mine told me he knows because I:
Don't make an effort to talk to people and can be quite cold, but to her I am warm and make an effort to talk to her and make her laugh
I smile most with her
It's quite clear I'm most happy with her company
Look at her a lot
Mention her name more than anyone else's in conversations