We've got 14 and 15 year old comedy writers on here?! That's cool. I remember when I was at school that I used to always try and inject comedy into my english work.
Anyway, if you're stuck for ideas then I suggest you give 'brainstorming' a try. Think of a situation that your characters could be in and generate as many things that could happen. From there, then branch out to what emotions they could be going through and what could happen next. You'll be surprised at the amount of ideas you generate.
Oh yes, and write, write, write. I must have written about 12 sketches this week and 11 of them were awful, but one was a real gem.