If it seems like a bit too much, say something.

We're not doing anything formal for either of our teachers this year. Last year they asked for a $20 donation.

I try to do little things throughout the year to let our teachers know how much we treasure them. We bring in doughnuts and flowers. I ask what they would like for their classrooms. DD's teacher said that the best gift she got this year was 3M packing tape because the stuff the school has is so cheap and hard to use!