I agree totally with what cory has said.
food issues are not the same as food fussiness.
OP, if your child really does have food issues, you need to raise this with the school.
eg, my dd1 has food issues (really, honeslty, major ones). one of her biggest is to do with drinking, and yes, she was offered only water as a baby and toddler (aside from milk, of course!)
when she was 12 months old, her first drinking strike occurred. for 4 weeks, she did not drink a single drop of fluid. I stayed firm, offered her just her usual drinks - water or milk. for 4 weeks. in a heatwave (I made sure she was getting plenty of liquid foods, soups etc, so she didn't dehydrate, thankfully).
she slowly began drinking again after the 4 weeks, but noithing like before.
when she was 2.6, she stopped again. she hadn't really ever recovered to drinking a normal amount, and this time she stopped for 9 months.
It was a nightmare.
It has affected ehr (and our) whole way of life.
It took us about 2 years after she started drinking agin to get her to drink more than a sip a day. and htis was with offereing just about any bloody thing she damn well fancied.
we worked for years to get her to drink pure juice. and then we worked on reducing the amount fo juice.
she is now 6, and yesterday she probably drank about 100ml all day. the day before was a good day, and she drank about 350ml.
if the school's policy is going to cause your ds serious issues, then tackle it head on. but do it nicely. explain his issues.
and if the school still don't listen, try to get a gp letter or similar stating it is necessary.
honestly, this healthy eating/drinking thing is all very well, but sometimes it can cause real harm.
dd1 stopped eating lunch at school because a cup of water was put down at her place by mistake. it took her over a year to build up to eating there again. so that was no food or drink all day.
bloody marvellous.