Being underweight during your major years of development is dangerous for both boys and girls. If you are undereating it's likely that you aren't hitting your RDI of macronutrience which will cause long term damage to your body and in particular your bones.

If you are a chronic under-eater, simply 'eating a lot' may not be enough because 'a lot' is subjective, it might still be below your TDEE. It would be worth calculating your TDEE and counting your calories so that you KNOW that you're actually hitting your goals. Don't focus on stuffing your face, make smart food choices. You may have a small appetite so eating more calorie dense foods would be beneficial. Don't skip meals because if you don't have the appetite it can be hard to catch up later in the day.

It might be worth also talking to your parents or guardian about your diet and concerns. Ask them not to buy 'light' versions of foods. Think up some calorie dense meals that you can keep in the fridge and heat up through the week and take to school. Make smoothies with nuts, full fat yoghurt and milk, avocado etc (shakes are a good way to get a serving of veggies in too if you're like me and hate eating them). Add a bit of olive oil here and there to your meals. Have some bread with peanut butter lathered over it as a snack.