Talk to your coach a bit more. Generally, the coach isn't looking for full effort on every single pitch. Usually, a coach will want you to work on your control and off speed pitches to begin a bullpen session and then only after you are completely loose have you let it go and try to throw full speed. The full speed pitches are necessary to teach your body to throw harder. The result will be a little better velocity on all your pitches and just as important the ability to bring the high heat when the situation calls for it. If you've been hitting the corners at 72 mph and you go high and tight at 76 mph you have a pretty good chance of fooling most hitters. If you throw all your pitches at 76 mph you have a pretty good chance of watching balls go over the fence.
My son is younger so the number of pitches might not be the same for an older kid but once he's loose we have him throw 4 pitches about a foot off the outside corner, then 4 pitches about a foot inside, 4 pitches on the outside corner, and 4 pitches on the inside corner. He then throws from 5 to 10 off speed pitches. Next he throws a couple medium speed fastballs and then from 5 to 10 fastballs (depending on how the arm feels) trying for max velocity without throwing a wild pitch. After that if his arm is OK we'll throw to 1 or 2 simulated batters, mixing in each type of pitch.