my colleagues often tell me something like this: "You can't get the kids you train to gain weight." (they are 14-15YOs) "I couldn't gain weight when I was that age and lifted weights hard."
when i question them about 'hard' they almost invariably tell me about some body split routine like this. you want to get big and strong? SQUAT, Bench, pull ups, rows, deadlift, OH Press and cleans.
3-4 exercises a session, 3 sessions a week. 4-5 sets of 5. Heavy enough to make you work. Example:
day 1: Squat 4x5
BB Row 5x5
day 2: Deadlift 4x5
pull ups BW to failure 3sets
day 3: Squat 4x5
Then mix up the exercises the next week, just use the same basic template. Never leave out the squats. I might skip a week of deadlift once in a while and use cleans in their place...oh yeah, eat. If you can do all those exercises in one day (Monday has 8 exercises alone!!) you aren't loading enough iron on the bar. KISS. oh yeah, eat.