I had the same problem in highschool… and not that I am the poster child for gaining muscle mass or anything… but for me, when I actually started eating enough (I always thought I ate a ton… but didnt really) and actually started training hard (10 sets of curls and a 3x10 on the bench with some flies thrown in for every workout doesnt cut it…) I made good gains.
Whether or not you think you eating a lot and training hard, you probably arent. As some of the others have suggested, provide us with more details of your training and nutrition. Keep a food journal for a week and record the times and quantities of EVERYTHING that goes into your mouth.
your highschool football coach might know a lot about football, but I assure you that he doesnt know as much about beneficial weight gain and strength building as lots of the people on this site… 90% of the time, you will get some good advice here… follow it