No. I don't think that throwing 100 fastballs every other day or every third day is the best pitching program to develop a 90 mph fastball. I'm not sure if it's even safe.
Major League pitchers, for example, don't even throw 100 max-out fastballs every third day. They throw in games every fifth day (with some different types throwing in between, of course, like long toss work, bullpen work and PFP drills).
A better program to develop good velocity may be to throw daily for at least 10-or-so minutes, extending to 120 feet. Once a week, you may try long tossing as far as you can to break up the monotony of the 120-foot throwing performed on the other days. Twice a week, throw a bullpen not exceeding 60-or-so pitches. (During the bullpen session work on hitting your spots; work on your mechanics; work on your release-point and throwing "through" the target.)
Just some suggestions.
Your best bet is to talk with the high school coach at your school. Ask him to watch you throw a side. That way, you can get a little more hands-on-guidance about how you should next proceed with your development.
At 15 years old, I was throwing 73-75mph (and I threw 93-94 mph later in my career). Low-80s is WAY ahead of the game at your age -- which is why you really need to talk with the high school coach at your school so you can get some direction in your continued development.