I don't know how it works at your school but in mine Head Girl (which I am) and Head Boy are part of the Management Team....Which is basically ten of us who organise the events for the sixth form; parties, the leavers ball, design and create the yearbook, fundraising events, buy things for the common room/sixth form block and organise fun events for the sixth form etc. We also pay for all this ourselves through funraising (we started without a penny and have had to work hard to raise money for all this). A handful of us are also expected to help out at school events (parents evenings, presentation evenings etc) and Head Boy/Girl have to always be there to be "front of house" as it were...with generally one of us making a speech.
If this is what your school is like then...not to scare you off...but I warn you that it is a lot of extra work to handle on top of your A-levels. At the start of the year it's ok but as it gets to round about now it can be a lot to deal with...especially if your hands on like I am, I mean theres 10 of us in our group yet there are at least half of the team who never are active in doing anything, more often than not the same 4 or 5 people end up doing everything and it can be a drain! You wouldnt think organising events would take as long as it does! lol, but I'm sure your school is much better than mine!
And also...if the only reason you wanna be Head Boy is to look good on your uni app (which yeah it does look a bit better if you are than if your not) then remember...there's thousands of Head Boy/Head Girls out there so it's not always as prestigious as you'd think lol. But if you actually want to make a difference then go for it!
I just thought I'd share my experience ... like I say I'm sure that your school if much better than mine. Best of luck!!
Nicola x