Oooh, 13?! Such a fun age. Wanting to be a grown up, but so not anywhere near close. haha.
Ferris Buellers day off. Might be a bit old for them. Pitch Perfect (if they like singing, it's funny, set in college though).
I second Clueless, it's fun. But again, might not translate well since it's over 20 years old...
These would totally work:
Easy A (it's a take on the Scarlet Letter, it's really funny)
Bring it on
Princess Diaries (I loved it and still love it!)
Warm Bodies (zombies, but it's cute and funny and not scary, take on Romeo and Juliet)
I just realised, these are all set in America. Sorry, I don't really know any British ones...
Maybe St Trinian's (2007)? It's a public all girls school with cliques and all.