I am not a teacher, but my next-door neighbor is. She likes gift certificates. It doesn't have to be $50...even $10 is nice to have. She also likes things that a teacher might need to teache...pencils, pens, crayons, papaer etc. There are a lot of things that are not supplied by the school system which teachers then have to go out an buy for themselves. Or how about a gift cert to an office supply store, where they sell that stuff...then she can pick what she wants.
My neighbor likes bookstores, getting her nails done once in a while, a pizza or chinese food occasionally, grocery stores, clothing stores.
She also would appreciate a heartfelt thank you note with no gift attached, written by you and your child, perhaps outlining how she made a difference.