Au Revoir Les Enfants (especially) and Europa Europa are both very good films, though serious (both set in WWII).

For something more light-hearted, try My Life As A Dog or if you can find it the excellent Czech film Elementary School (Obecna Skola), though both films feature children a little younger than 12.

If they want something funny, try Delicatessen - it's a film which ought to appeal to most children from 12-15, provided they have enough appetite for the gruesome (which in my experience is usually the case).