If I may.... may I suggest that you check on the possibility of getting extensions if you need them (as amarynth suggested) and then make a decision? I don't know for sure if this works the same way in NZ, but if it's possible for a case to drag on for an unpredictable period of time... well, they do need people, sure, but I seriously doubt they would want a long-running trial to cause you to fail courses.

Not that I'm trying to be a wet blanket, mind. I applaud your interest and responsibility; I've no doubt that you will have another opportunity, preferably with better timing, and it's not as if you'd be shirking shirking -- just helping on one occasion instead of another.