Homemade goodies I think depend on your school district whether the teacher would trust them. My SIL works in a pretty bad school district, and says any homemade good she receives go straight in the trash when she gets home, as she's had to do plenty of home visits for her job and doesn't trust eating anything from them. My school distric is pretty nice, though, so I don't think our teachers would throw out goods unless it was something they just genuinely didn't care for. I sent in chocolate covered pretzels with gourmet sprinkles on them, with target gift cards for my son's preschool teachers.
I think a gift card with something small, like cookies, chocolates, etc is a good safe bet.