Hi
My info is old now(13 years ago) but when we moved into a small village and enrolled the children(a big help for the school too as they need the numbers) the school went all out with intensively teaching them Catalan for the first six and 12 months. They took our teens in for six months and then moved them to the nearest high school and all the kids in school ended up getting the language very quickly. It may be a little different in the big city : ) but I'm sure they'll do all they can to help.
Bruixot
James