Make sure to do some lifting. Doesn't have to be a super smart program, just free weight compound lifts gradually increasing the weight, maybe doing some kind of periodisation (lowering reps over time)
Also make sure they are gaining weight if they are too light. Most drafted hs hitters are 175-200 in these days so if they are sub 170 eat enough and gain some weight.
With the hitters I work with I usually do a 3 phase program. First 6-8 weeks is usually mostly movement and mechanics, then I do a batspeed phase with ol ul stuff and also med balls and then do a more challenging phase working on handling velo, spin, plate coverage and so on doing stuff like short box, higher velo machine and ideally some live ABs.
Also make sure do do some med ball and stability stuff.
Also some throwing program of course but I dont know that much about this stuff.