CT had some suggestions for improving strength endurance last night in his Prime Time thread:
If you respond best to higher weights and lower reps, that's how you should train (I'm like this myself). When you want to improve muscle endurance you can either:
1) Gradually reduce your rest intervals (something like a 15 sec. reduction every two weeks) and do more sets while keeping the weight heavy.
2) Use rest/pausing techniques like extended 5s... use a load you can lift 5 times, and do 10-12 reps with it, taking 10 sec. pauses during the set when you need them. Do not reduce the weight.
3) Use GPP work (sandbag, sledgehammer, wheelbarrow, farmer's walk, sled dragging) with relatively heavy weights performed in "sets" of 1-2 minutes.
It might be worth you looking up Joe DeFranco's site and/or articles as I believe he has done lots of work with HS players and is an expert on passing football tests.
I hope this is of some help.