TBH I do think you're overreacting. A teacher did something kind for your child. If it happened all the time that's one thing, but she's unlikely to make a habit out of it since it probably came out of her own pocket.

Your points are all valid, but you've got a lot of years ahead of you at this place - if it were me I'd suck it up, thank the teacher if you see them (or send $1 in with your son tomorrow) and start packing more food (veggie sticks?), or label food in a way that he knows what to eat when. You don't want to become known as the mum who got angry with a teacher for doing something kind.

Sometimes people give my kids food that in an ideal world they wouldn't eat. But I would rather my children experience being shown kindness than be the lolly police.

FWIW i think you are sending plenty of food... Growth spurt? Parasites? Is he cold or bored?