Just to chime in I was 5'8" 125 through my jr and sr year in HS and could anything and didnt gain anything. My first yr in college I was actually up to 130. But I really needed to gain some weight so I ate and ate. But I didnt eat right, junk, crap, and beer. Sure I got up to 155 during by Soph yr.
But by my Jr year I was gaining too much, I was over 165 and climbing and not lean. I was working out allot but not much cardio, and as I look back, still not eating right. Too much beer too. So like the others have said eat and eat, but read the articles here, theres tons of great info for hard gainers. But make sure you eat smart and you will make great gains.