When my daughter was really small and attending a public school, her teachers would give her little hard candies as a reward when in the classroom. This did not sit well with me, since I did not give her candy at home at that time. Even on Halloween, her Trick or Treat candy would be made into art on a poster board. For several of these early Halloweens, she wasn't aware that this stuff was for eating.
The deal I struck with my daughter, was that she brought home these little candies and each candy gave her 5 minutes of her "currency," a special activity that she loved to do. If she chose to eat one of those candies, well at least then she was learning to choose, just like the opportunities she had with other choices and seeing how they felt. As for me, I felt so good about her not eating all that candy and that each candy at home gave her 5 minutes of the special thing that she loved with me. I highly recommend that parents take control of things that are going on in the classrooms that go against your ideas of natural immunity and health in a loving way. Our deal did not interfere with the way the teachers wanted to do things and their poison didn't harm my child's teeth or health as it could have without my involvement.
So many teachers give candy as a reward. I've experienced the challenge they face daily with neurologically damaged kids with behavior issues. There is no threat that works and candy does get results. I understand. Teaching is alongside a circus act where a teacher uses smoke and mirrors and candy to keep the herd focused and calm. Since I couldn't change what was happening, I felt pleased that I was able to turn the experience into a "currency" situation for my daughter. She always received 15 minutes every day of this special thing, but she was in a position to purchase extra. Candy changed from reward to currency.
Being a well-behaved child, you can only image how much candy she'd bring home daily since I only allowed one piece to equal 5 minutes! Instead of fighting teachers who will not change, work with your children around what they do.