Three days a week my daughter goes straight to gym from school, so her lunch box looks like its about to explode! She too is on the thin side, with all the workouts, its almost impossible to put weight on her. Here is an example of foods i send for the day.
Breakfast= mini bagel with pecan nut cream cheese or Protein Cheerios with milk 12-18 protein( she loves them)
snacks during school and gym= Salami, cheese stick, crackers, whole fruit leather, banana, apple slices, fruit / veggie pouch , drinkable Chobani yogurt 14 grams of protein in those, celery with peanut butter with a few dark choc chips
lunch- pb&j, turkey/cheese sandwich, roast beef roll, egg salad sandwich, chicken pasta salad, chicken skewer, tortilla chips and guac, hummus and pita
LOTS of pre workout fluids too .. I send a hint water ( drinks during class) for lunch a small gatorade or body armor and then of course a 32 ounce jug for gym.