Summer students disrespectful of my home.

More twit than wit.

I saw all the OP's 'wrong wrong wrongs' and thought SHE was the 15 year old.
Couple with her apparent lack of common sense. All she needs to say is what @Sidebeforeself has stated. Simple. Factual.

And these students are only here for 3 weeks. Good lord.

Are you sure you're a grown up, old enough to host children OP?

The language school that I host for tells us to regularly check the rooms in case of any damage or other problems. It’s much easier to sort things while the students are here than to wait until they have gone home. They know the rooms will be checked so it isn’t an invasion of privacy at all. In this particular situation I would tell them not to take food to their rooms and to put any food waste in the kitchen bin. I bet they wouldn’t leave food waste in their rooms at home.

@Zone2NorthLondon but they are in her home, therefore her rules apply. My kids friends may be allowed to tramp their muddy shoes upstairs in their own houses, but they’ll take them off at my front door. they may eat in their own bedrooms, but in my house they’ll eat in the kitchen. They may swear in their own house, but they won’t in mine.
do you let child guests behave as they like in your house?

Why are you being so arsey? Just say “ Please don’t let food rot in the room..here’s the bin” I don’t see what the problem is that you’ve come he to resolve

Tell them to put uneaten food in the bin. Don't worry about the lunches. If you're so certain about what's Right and 'Wrong wrong wrong' and disinclined to listen to other opinions, why bother asking advice on a forum?

That's disgusting 🤢

If they are only there for 3 weeks I wouldn't even go in there tbh. It's their space for 3 weeks you can give it a big clean once they are gone. Leave them alone and let them eat what they want. Just say NICELY please don't leave any food in the room.

When you say you’re worrying you’ll over react with them, my advice would be… just don’t!
You can bring it up without over reacting or upsetting them. Just say something along the lines of “girls I’m loving having you here, I hope you’re having a great time, thank you for being amazing house guests, can I just ask one thing of you though, please can you make sure any food waste/packaging goes in the bin?” Like you say, they’re young, they won’t have thought anything of it. They sound like good kids. It doesn’t need to be blown out of proportion :)

Well I mean it could be worse… we had a student from China come and stay with us once and we couldn’t work out why her room smelt absolutely awful. When we went in to clean it we opened one of the drawers and found a whole chicken carcass that had been stripped to the bone 😳 she’d been picking at it over several days. How she didn’t become seriously unwell I will never know. (And we were providing cooked meals etc etc).

And what does that add to this scenario
presumably your familiar & familial group eat in the kitchen
They are exchange students away from home,her home,her rules aren’t their Norm and are unfamiliar

If they’re leaving it to rot in their bedrooms they don’t like the other food either.

Just tell them to knock it off. The first stop when they get in is the kitchen, bags are emptied and food goes in the bin.

Its three weeks. Take a deep breath, it’ll be over soon.

They're French children. Pastries, good quality but slightly stale French bread sandwiches. Croissants with full cream hot chocolate. Give them the French equivalent of junk.
Just don't let them eat in or take food into their rooms. No need to be cross, or they will be even more secretive.

Have you asked them what they want in their packed lunch? What do they eat for lunch at home?

No it’s a huge invasion of privacy.

Food is eaten in the kitchen in our house. The end.

I don't think they have additional needs. They are young and preoccupied/ lazy.

A simple " Please don't leave food in your rooms, use the bin, Thanks"

If it's "NO PROBLEM" for them to have food other than what you provide, why on earth did you say in your OP, "Im annoyed they have someone else’s food, and im annoyed because it’s disrespectful to my home"? 🤔

OK - but you wrote in your OP - "Im annoyed they have someone else’s food," - which makes it sound that this an issue for you

Just remind them to use the bin. If you're going to get so worked up about them eating food you havent given them, perhaps hosting isn't for you.

You are not laid back if you’re there looking at every piece of rubbish to work out if it’s your food or not!
Either they don’t like your food or you don’t give them enough and they have to buy more.

”The kids having other food IS NO PROBLEM.
i MENTIONED IT. ”

I’m annoyed they have someone else’s food.

Your food is evidently not fabulouso, they’re not eating it. They’re actively seeking other food that they bring back
Offer them alternatives seeing they don’t like your food eg yoghurt,pastries,fruit. Just replicate what they obviously really want to eat

" Im annoyed they have someone else’s food, and im annoyed because it’s disrespectful to my home. "

"I’m so annoyed I’m afraid I will over react with them. "

You are over-reacting. Do you actually have children of your own because you arent acting like it. And have you actually asked them what they like to eat? Of course trying different food is part of experiencing another culture but sometimes you also need to be considerate.

Mmmmmmm….did they know you were planning on cleaning their rooms? I suspect not. They may feel you have invaded their privacy.