There will be a LOT of variation. My experience is in Massachusetts (USA) where breakfast and lunch are currently free (fully subsidised) for all students.
Breakfast is typically at least cold cereal, granola, etc. with milk, yoghourt, fresh fruit, muffins, bagels, toast, cheese, spreads, juice, milk, etc. Maybe more hot/fresh cooked things in a larger school, a small school may only have take-away boxes or bags.
For lunch, where I am there is a main dish (with a veggie option - may be separate or a version, like "assorted calzones" where some are meaty and some are meat-free) plus a "deli station" (DIY sandwiches) and a salad bar. Then there will be at least one steamed fresh vegetable, some kind of starch like potato or rice or noodles, and fresh fruit. Drinks are water, milk (has to be fat free or 1% but includes flavoured milks) and fruit juice. In the high school (age 12+) there are vending machines with juice, energy/sports drinks, sparkling and still water, iced tea, American lemonade, etc. - but Coke, etc. is not allowed.
I don't think the main dishes are all that healthful - it's a lot of chicken, beef (almost no fish or seafood, and I think no pork or ham), burgers and fries, calzones, pizza, pasta "casserole" things, DIY tacos, various "bowls", etc., and they repeat a lot. But the sides are well thought out and nutritious and the deli and salad bars are really nice. I've never heard of anything spoilt or burnt; it just wouldn't be served. Everyone involved in school meals has to have a state-administered children's nutrition and health certification. There are state standards on nutritional balance, calories, etc.
There's also a cool "farm to school" program, Harvest of the Month, which is done state-wide, where they focus on one local crop every month (pumpkins, peaches, cranberries, cucumbers, wild rice, etc.) The cafeteria serves dishes made with that crop locally-grown, there are posters and info sheets and stickers and such, and the high school does an optional informational session for the students with someone like a local farmer, chef, etc. about that item, and students have a chance to share recipes, photographs, creative writing etc.