Hum.... not so sure about the state system. Right now, there are massive cuts and in some classes, there are up to 42 pupils for just one teacher. I'd rather go for the concertadas schools, which are half state and half private. Have a look a this site for more info about living in Spain www.mumabroad.com/
I can only say that there is a massive squeeze on education, along with everything else, in Spain due to the economic crisis.
We have recently returned to UK after 8 years in Spain when DS finished his Bachillerato at a state school to attend Uni in UK.
Obviously, as you are thinking Valencia, learning Valenciano will be compulsory but not difficult for them as they are fluent in Spanish already.
Sorry don't know anything about schools in that area but if you can find out some of the names of those you are considering they might have a website (our sons did though it was never up to date!) which might give more info. If you think I might be able to help with any other questions let me know. Good luck.
If we can sell up here we plan to move to Spain and I have no idea what to look for in a Spanish school. I understand their state system is good. DDs are fluent in Spanish so language isn't a problem. Google hasn't helped, maybe I just don't know what I'm looking for, if that makes sense