ok, so I googled "birthday treats for school" and did the images. A couple of ideas....
-Twinkies with a printed wrapper on each.
-Print a piece of paper "thanks for making my birthday s'more fun" make "s'mores" by taking graham cracker smear with fluff (or use Kraft stackable marshmallows, or even PB if that is still allowed) put a chocolate bar then the other half of the graham cracker. Put in baggie and staple the note to the front.
-There are actually more than a few ideas where you put different things in a baggie (a starburst, a kiss, a marker, a rolo) close the bag and attach a printed note to it.
You are a much nicer mom than I ever was! The only time I did anything like this was the few times that DS's birthday fell on the weekend when they had church camp. I could be wrong, but I don't remember ever taking stuff to the classroom for birthdays. I did make some wild cakes for their small gatherings of friends/family at the house though.