Try eating 5 or 6 smaller meals a day. This isn't as hard as it sounds, but it does take some dedication. Using a program/website like FitDay can help you calculate your maintenance level for calories. Lets say you burn 2000 calories during your day to day life: sleeping, digesting, doing homework, working out. You want to eat about 300-500 calories over maintenance a day to gain weight healthily. Start with 2300, check your progress in 2 weeks and kick it up if necessary.