When some talk about pitches that are mastered, I think many people look only at the end result of where the pitch ended up, not necessarily how the pitch got there. One of the things that gets lost in the shuffle to learn breaking pitches is the fact that the fastball is the foundation that all other pitches are built on. If the mechanics are flawed in the fastball delivery i.e. short arming, chicken winging, crow hopping, etc. you will have inconsistancy, unnecessary wear on the body, and/or potential for injury. These flaws in your fastball will follow into all the other pitches.
An effective change-up looks like a fastball, the arm has to go around at the same speed, the footwork is the same, some even say that it is more effective if the batter sees the ball at release as opposed to the back of the hand, and the ball should have a spin on it as well to fool the batter. Now, spot that inside low and outside low, in and out of the zone, and it will be deadly in HS ball.
Same thing with breaking pitches, for example: if you are stepping way outside the powerline to get your screwball to break, you are tipping your hand to the batter, and possibly setting yourself up for an injury, or an illegal pitch. Granted, there are not a ton of batters in the HS level that will pick up on these little "tells", there are a few that will. We had a girl that pitched for us at 10U that would slap her leg with her glove on fastballs, but not on change-ups, you would be suprised how many girls and teams picked up on that even at 10U.
Fastball and change-up as described, and the addition of a good breaking pitch. Probably all any HS pitcher will ever need.