I'll give you some examples, simple stuff and you probably know it but this is how we feed post op/injury patients to get their weight and muscle back on and incidentally is also how top level rowers here eat. Every time you work out have a protein shake/milk and a muesli bar about 45 mins before you exercise, then afterwards eat a meal (try to time your workout so you eat within 45 mins of finishing). For breakfast eat something like eggs on toast/black pudding and beans/porridge and also have a banana and drink a big glass of milk or have some yoghurt. Scotch eggs (done in oven), cheese toasties, sandwiches and stuffed pita breads make good mid morning snacks and are easy to make and carry. For lunch try to have a 'dinner type meal' so meat and veg with some carbs like rice or more bread. Curries, stews, meatballs, stir frys, wraps etc can all be made in advance and last in the fridge for about a week so all you have to do is reheat (or eat cold) and add some rice, pasta, bread or quinoa or whatever floats your boat. Don't reheat rice that's more than 1 day old if you can avoid it because it will probably get you sick and you don't want to lose your hard earned weight gain down the loo. Find a muesli bar you like thats not too sugary and just always keep a couple in your bag and drink choccy milk or something like that when you can. For dinner, meat and 3 veg should be perfectly adequate, but if you're having salad as a side, add some rice/ mash potato/bread. Invest in some good tupperware so you can bring your big lunches with you to school easily and on a Sunday, help your mum cook up some big batches for the week and you will find it super easy.
As has been said you will bulk up as you get older anyway but if youre dead set on doing it now then hopefully this helps.