An apple wouldn't send his sugars to the 20's!
Have you seen the **** that school kids eat? Go to your local corner shop/garage in the morning or after school and watch what the kids buy. Cans of full sugar coke, big bags of Haribo, Mars bars, pack of chocolate peanuts...it makes me ill to watch.
Your boy is NOT just eating an apple, believe me. He is eating the absolute rubbish his mates/peers are eating - he doesn't want to be different, he doesn't want to be seen as weak or an outsider because of his diabetes.
Primary school is different, the kids have so much more care, support and supervision. Secondary is far more feral, unfortunately, and he's having to run with the sugar addicted pack.
He thinks like all teenagers, that he is immortal.
I don't know what to say to you to try and stop him, but this is undoubtedly what he is doing.