My son is extremely under weight. The nutritionist has us calorie packing all of his food. For example adding peanut butter to his bread, a little cream to his milk (and no water). We still avoid sugary food and soda though.
The doctor also has him on a strict eating regiment. No snacking between meals so that he is hungry at meal time.
This is advice for a toddler, so the more I think about it you should probably go see a nutritionist if you are that rail thin.
Good luck with this and with school.
In the long run that ADHD medicine will slow down your metabolism. Watch.
I had the same issue with my ADHD medicine just try to eat as much as you can when you do eat and drink a lot of water cause the medicine ruins your appetite