Hi,

The important thing you need to remember is that for children in school the first language is Catalan, ALWAYS!
Spanish is OK for us adults, but the kids have to learn 2 new languages.
I dont know if it will be the same in Tres Calas, but in Terra Alta, another comarca (county) of Catalunya, children of this age are not allowed to progress to secondary school until they have a pretty good "handle" on Catalan, they do intensive courses during school hours and also after school, to bring them up to scratch and fully prepared for secondary school.

You also need to register your child before the forth coming term, or you sometimes have to wait until the next one, which can be another 4 months.

Also make sure that you have a fully documented record of all health issues including vacinations, they are very strict on this.

Hope this helps.