Dear Carmel Huxtable
Your sister can do two things.
She can take good food from home so that she doesn't have to buy the canteen food.
But even more useful, she can start to lobby people who have power over what happens at the canteen.
The best way to do that is to get a group of people together who'd like to have some alternatives to what's in the canteen now. Then they should work out what things that they'd like to have there. When they're ready they could put a proposal to the parents and friends committee or to the principal or whoever has the authority in the school to have things like canteen items changed.
If the kids can show that they would like to have healthy food in the canteen then people will listen to them and support them.
Lots of schools are interested these days in improving what they have to offer kids.