Martin, firstly, I would ask Geoff Poole your former Deputy Head if he's still around, he had a marvelous set of exercises that emphasized fitness whilst working hard on the boys technical skils. I have used them for thirty years - thanks GT. they were very similar to sessions I have observed professional football coaches use to improve skills and concentration under pressure, in this case tiredness.

If you are going to do fitness work then place the emphasis on 'Learning to Train' look up LTAD on the internet. Remember with a group of this age you will have some near ful grown and some who are still pre-puberty.

One of the most effective strategies I used was to run the school cross country club and sign up all the hockey players. They won a number of medals for X/C and four ended up as hockey Olympians.

In the limited time you will have available, I'd place the emphasis on hockey, if the boys are technically reasonably sound I would utilise conditioned games. Dr Karl Stagno has researched the subject and found conditioned games raise the heart rate into the training zone very efficiently.

For example:

1. Play 3 v 3 on a 25 yeard by 15 yard pitch - score across the entire goal line.
2. 5v5 (7v7) No goal can be scored until the entire attacking team are over a given line (place two cones on the pitch).
3. Every player must defend If the entire team are not in their own half and a goal is scored then all the defending team run a suicide (be sensible here) at the end of the session, players soon encourage team mates to do their fair share of the work.

Players will enjoy playing rather than running without the ball.