Students are given food at five points throughout the day: 3 meals and 2 snacks. Snacks are provided mid-morning and either mid-afternoon or in the evening depending on the school. These mainly consist of cookies and cakes. Pupils spend their mealtimes in different locations depending on the school - for some it is significantly house-based while others may have a whole school cafeteria. In general, it is fair to say that food in boarding schools isn't at a great standard. It is therefore important to bring snacks - pupils have tuck boxes  (https://www.mossman-trunks.co.uk/31-school-tuck-boxes)   and sometimes have access to a fridge. Most students bring sweets, crisps and pot noodles though almost anything is allowed. It is recommended that students bring lots of pot noodles both to eat and to trade. Should a student eat a lot, and be in a position to do so, they may be able to bring meals ready to be stored in a fridge and heated in the microwave when they get hungry. 
School breweries - mini kitchens throughout the boarding houses - contain cereal and toast that students may consume throughout the day. There will also be fruit somewhere in the house, though by and large it will not be in the best condition.