also, may depend on the subject. i've heard for medicine and stuff they're very hot on "leadership" qualities - a number of friends got asked at interviews "when have you last demonstrated leadership qualities" etc, so being a senior prefect at school is a good example.
But, if it's an acheivement at your school, I would definately pop it down - it can't hurt, and other unis may love it even more. As with all things in P.S.'s drag out the skills - i was a House Captain, and i drag out of it leadership, organisation, and just to show the rounded aspects to it (sport etc). But, it's true that when it comes down to it, Cambridge won't really care too much, but other unis, especially the ones like Durham who are looking for random reasons to pick people (with no interviews and huge application numbers, it seems to come down to that pretty much)