Teachers shouldn't be anywhere near your dc's lunch box - we are having our own lunch break then, thank you very much! 
Lunchtime supervisors probably can confiscate if the school has a policy about items that can't be included.
My thinking on this, as a teacher & a parent, is: school dinners in this country are generally absolutely appalling. That needs sorting. Once it has been, schools will be in a credible position to tackle the not insignificant proportion of packed lunches that are also appalling.
Meanwhile, I send my dc in with pack ups that meet their school guidelines. It's not actually difficult - just no nuts (allergy reasons), no sweets/choc (fair enough - no-one needs those during the school day) & no drinks that aren't water in water bottles (but fine to send in cartons/bottles/cans of juice, so I don't know why I don't get to reduce the packaging waste by pouring juice from a litre carton into a re-usable bottle. Still, their gaff, their rules).
I'm about to move, kids'n'all, to take up a teaching position overseas at a private international school, & they're absolutely taking no prisoners on lunchboxes. It's protein filled sandwiches, water, as much fruit & veg as you like, & that's it. Non-negotiable. No crisps, no cake, no sweets. They are for weekend treats. You don't likey, you buy a school lunch - which is also remarkably stodge-free by comparison with the UK equivalent.
I suspect both the dc & I will be considerably more svelte & tucking into our evening meal with enthusiasm
.