Our school has a policy that staff cannot receive a gift above $25. Even though this is from several students, your gift card would fall under this catagory. Maybe the problem wasn't the idea, but the fact that it was sent home with all the students from school and therefore interferes with school policy. Had you contacted parents personally, that would be different.
I have 3 school age children and I have always done this for my children's teacher. I send a letter to each parent and advise to make their shopping easier I would be willing to take a collection from anyone wanting to participate and pool the monies together and present the teacher with a SCRIP certificate. She then is able to turn in her order for a certain amount and she chooses the merchants of her choice. Some years 1/2 the class participate and some the whole class but what ever the percentage is the whole class signs the card. One teacher even called me to find out who contributed so she could send them a thank-you and I told her I don't keep up with it so she sent one to each student! The teachers seem to really enjoy this and if the parents don't want to participate, that's fine. I just feel during this hectic time, one less gift to worry about is a time saver. Our principal has never complained either.
FYI.....We just asked our principal this last week and she had a similar response. We asked if a small collection could be taken from each classmate and buy the teacher a gift certificates instead of her receiving 25 'little' unusable items. She said no because people would feel force into participating if they could not afford to.
We don't have a policy like that in place at any of the schools my kids have gone/go to. In fact, in most cases the room parents have done exactly what you have, although it's usually for an end of year gift rather than a holiday one. It has worked very well.
Our PTO has a budget for volunteer and teacher gifts. Plus the kids gets the teachers something and the children either draw names or play musical gifts.
Hmmm...that's different. I assumed someone complained. If no parents seem to mind, and in fact support it, I would try to gather up support to have them change this policy. Unless they're going to send out notices absolutely forbidding gifts, people are going to send them in anyway. The teachers might as well get something useful.