My dd, is 4 and started in September at a state school in Barcelona. As mentioned above it is all Catalan, except for 2 hours a week of English. No Castillian Spanish until they are 6
The pre-school years are very play-based here. They seem to spend most of the time drawing a colouring, singing and making things out of plasticine. Her class, and in fact the whole school is decorated with artwork, and they always have different projects on the go- the environment, oceans, different holidays, etc.
I've been very happy so far. We had a slight issue with a boy in dd´s class repeatedly hurting her, and it was dealt with quickly and efficiently with all parents involved.
There are 25 children in the class and there are always two teachers in with them. This seems standard in our area. There is a big problem with places here in Barcelona, I don't know if that is the same elsewhere in Spain, but many parents have not been able to get their children into the school of their choice.
Children are a lot more boisterous than in the UK. Dd was terrified of the children when we first got here and now she is very much one of them.
Dd´s school has a very good reputation. But just like the UK, there are plenty of not so good schools. My biggest complaint is lack of sports/music facilities for when they are a little older, but parents tend to enroll their children in after-school classes for those kinds of activities.