In general making sure they are strong in a variety of differet rep ranges (all position players that is) will be beneficial because optimal strength is being strong all over...not just in higher rep ranges (endurance) or lower rep ranges (strength). Undulating periodization could really help you here as it will help the kids work with a variety of rep ranges over a number of weeks.
I would probably use a 3 day total body routine and really focus on hammering form. Then, as practice starts i would drop it to 2 days a week. Also, make sure you are using compound movements as the bulk of your program. Some sort of dynamic stretching and mobility work can also be benefical as it will teach the kids how to warm up their bodies and prepare for movement before games, before weight training, before practice or as a resorative measure on off training days. Teaching them some static stretching (and possibly SMF if you have foam rollers) will help them stay limber and help to combat soreness which can be a problem if they are weight training one day and then do position specific drills the next.
Do you know anyting about low level plyometrics? I say low level because most high schoolers are going to lack the strength in their hips and lower bodies to do anything that is really advanced (depth jumps, intense bounding). However, some low level plyometrics will help to teach them reactive strength, how to absorb and apply force, as well as help them be more explosive and powerful. Just remeber that plyos aren't for conditioning. There is a lot of force transfer going on there and using them as a conditioning tool can possibly lead to injury as they fatigue and try and apply mazimal force. Start them slow with easy low level plyos (like hops in place) and on a low end of touches per training session (say 60-80 toe touches per session) and slowly progress them as the offseason goes on. If you are using the plyos on days were they are weight training it would be best to set up the training day like this:
warm up- general (3-5min of jogging)
warm up- dynaic (mobility drills to loosen up hip and shoulder girdles)
low level plyos- 60-80 touches
cool down- stretching (static and SMF)
hope that helps.