The most important thing is making sure there is enough juice in the tank.
A lot of schools play very close to the line on this. The common practice of assuming that a certain plane will go for say 3 hours if full, and writing down the times in a flight log, and subtracting the presumed fuel usage on each flight from the presumed fuel left on board, is very dodgy and there have been a number of accidents due to this.
We've all read much about a high profile court case in recent years involving a Seneca, and that was a very good example of a number of factors, dodgy industry practices included.
The important thing is to not accept the school's word for the fuel on board (FOB), and always do a physical check, and fill right up if unsure (even if you get a bollocking afterwards for having come back with too much fuel to carry the next load of obese instructors/passengers).
I've had two cases where I would have definitely come down had I relied on the FOB figure, which others routinely relied on. In one case the tanks were so empty it was amazing how the previous person made it back at all. That one would have been an engine failure shortly after takeoff, for me.