When my kids where young, I was a school volunteer and got to know quite a few teachers, personally. My daughter now teaches Kindergarten. Gift cards are wonderful but like others have said, choose WalMart or Target.
Other ideas -
A book for the classroom (appropriate reading level)
Small note cards (for teacher to thank kids/parents for gifts)
Post-it notes (large enough for teacher to write a note, to stick to a paper or folder)
Some schools have Teacher Wish Lists, so you might also want to see if your school has one.