Since the majority of the pitchers at my school are power pitchers, I want to go for ground balls. What are 2-3 good pitches to do this with?
Any pitch with downward movement (e.g. sinker, slider, change-up, curve, etc.)
Agree with Roger that you’re looking to throw pitches with downward movement. Something that helps in that regard is throwing over the top, which naturally creates a downward angle on the pitch. The HS sidearm pitchers (and even the 3/4 slotters) I’ve coached seem to have more difficulty inducing ground balls. I suspect they just aren’t generating as much drop.
Don’t overlook the top of the zone. Pop ups and swing / miss are good too. Bats are faster lower in the zone than at the extreme top of the zone. Faster swings down and in vs up and away.