Well there's a number of things here. Firstly this is a nice money making scheme the school has, £5 on snacks a water is quite a lot so I'm guessing the school are somewhat encouraging it. What 11 year old will want to eat some free slices of fruit when there are cakes, cookies, muffins etc all laid out?
Secondly, there's an response on another thread today about private school children not learning the value of things. I'll bet your DD has a least one friend whose parents give them heaps of cash and don't mind how they spend it. So you daughter is pressured to spend freely.
Thirdly, is she eating the lunch you are also paying for if she's filling up on snacks?
Fourthly, it's really unhelpful the school lets her run up a credit. I'd call and ask them not to.
Put £5 a week on her card and tell her once it's gone, it's gone. Talk to her about money. How much you earn vs how much things cost. How much a house costs , your mortgage and other bills, travel expenses, school fees, and running a house hold. Importantly, talk to her about average salaries, minimum wage, etc.
You could even say to her that if she doesn't spend her £5 per week and saves it, she could buy herself x by the end of term.