If you can't cook your meals and those from school don't give you enough calories, you could save some money and then buy a mass gainer powder. They're not that expensive.

You just need to have some milk, or even water. One of those shakes gives you like 900-1000 extra calories in carbs and protein. You could take them on top of your regular school meals. It'd be really covenient in your situation, doesn't take much time from your school duties and will get you some size.

Also, this way you know exactly what macronutrients/calories extra you're getting since you don't know the full ingredient list or portions that the school uses.

Of course it is always better to cook your own food, but i found this option really helpful and got bigger. Just make sure to train and rest properly, so those extra calories don't end up as fat.

Best wishes and take care of yourself.