I wasn't. They said only the ones who fit on the stage at assembly, about half of us.
Trouble was, everyone had to do prefect duties. Including supervising say, third years when it was wet in the dinner hour.
And as non-prefects we had no sanctions if anyone misbehaved, unlike the prefects who could give one order mark....
It wasn't anything like the way schools are now as regards discipline, but this duty in particular scared me stiff - and has left me for the rest of my life scared of trying to exercise authority.....