Check on your school district's gift policy first. Our state put a law into place a few years ago that screwed up gift giving. Group gifts from the class, and individual gifts over a certain amount of $ have to be reported to some state agency. The exception is if the gift is for the classroom, so some of the group gifts are now to places like Staples or AC Moore so that in theory, the teacher can put it towards supplies for the classroom (and reduce his or her out of pocket expenses).
So with that in mind, if I do gift cards I usually do a gift card to Amazon or one of the local restaurants that I know the schools sometimes order lunch in from, or Dunkin Donuts or Starbucks.
I also sometimes do a small gift box of gourmet homemade treats (chocolate dipped oreos and pretzel sticks, peppermint bark, shortbread, cocoa cones, tea, personalized iced sugar cookies and other goodies) along with something like a bottle of good nail polish (OPI) along with a note to take a break for some pampering during the holidays. If I do treats, I usually do them right at the beginning of December so they're not bogged down with baked goods already. These are kind of tricky though - I usually only do them for teachers who I know like sweets, have no food allergies, aren't dieting, or who I know have kids who will eat the goodies if they don't want them all. Those who I give them too rave about them so I know that they're appreciated but it's not the right gift for everyone.
This year for the teachers I know fairly well who I know are busy (those who are parents of young kids) I'm considering giving a "dinner short cuts" basket of my corn bread, rice pilaf, beer bread and biscuit mixes (packaged in decorative bags with pretty labels that have instructions) along with some kitchen gear like wooden spoons or dish towels because who can't use more of those?