I understand the cultural difference OP.
For breakfast, besides toast, cereal, crumpets etc. perhaps having some hard-boiled eggs, or Greek yoghurt in the fridge? This will help up the protein.
A sandwich can be very healthy and filling, especially with things like chicken (rotisserie chicken is great for this!), turkey slices, boiled eggs, tuna etc. These are all easy to buy and have on hand, they don’t need to be made from scratch, except for boiling the eggs.
I think ‘something on toast’ is always a good options for teens for a snack or light meal. E.g. avocado, cream cheese, cottage cheese, peanut butter, ham and cheese, beans, etc. It’s quick to prep and is really easy.
My friend, who has 4 teens at home, goes through 2 full loaves of bread per day, just on snacks!