That should really say in Catalunya as we are an autonomous region and so regulate our own school system, but here goes.
Rule #1 for any bureaucratic process in Spain,
Bring everything you can think of. We have been asked for expired passports upon occasion.
Bring 2 photocopies of everything.
You will need:
Llibre de familia if you have one,
you may be asked for a birth certificates with
an official translation and documentation through the embassy. Depending on where you are this can be a big hassle. We had to get the LONG FORM birth certificate. Then we had to send it to the Department of Foreign Affairs in Ottawa for them to accredit it's validity, then it was sent to the Spanish Embassy for a signature there accrediting it's validity and then it was sent to us. Once it arrived here we had to pay to have it officially translated, which meant sending it through the mail. They lost our marriage certificate once! WEE HA! Your embassy can explain the exact process to you for your country should you have to do this, don't leave home without multiple copies of the long form of both birth and wedding certificates. We'll be getting more this summer.
Marriage certificate with official translation and certification is also possible, though we weren't asked.
DNI, NIE or passports for everyone, both parents and the relevant kid
Certificates of Vaccinations
Empadronament, which is the certificate that you live in the place, you must register with the local city hall as soon as you have a place and this is also where you get this piece of paper.
Health card. (this is sometimes a hassle to get) requires all of the above, and you go to the CAP, which is the local health center to apply.
A million other forms,
Probably a pair of photos of the child, passport sized. They accept the ones from the booths for everything as far as I can tell.
Bring a copy from your bank of your bank account numbers - your bank should print out a sheet with the bic and number and I know not what else. People hand these out with gay abandon here, and that way they can deduct the money for excursions from your account directly. Seems weird, but there's enough bureaucracy here without adding to it for every excursion. I also sometimes took forms into the bank to get them to fill them out if I was lost enough. We love our bank and they are SUPER helpful.
Money. Though these are public schools, and nominally free, you always have to give some money for something or other, more to the parent's association, if your school has lockers, you have to rent those, and then there are the books. You have to buy them all.
There will be more forms at the school for you to fill out as well.
It may be worthwhile making a preliminary visit to ask whatall they are going to want before you arrive. Also if you live in a big enough place, there may be more than one school to visit.
Some school are harder to get into than others, and they work on a point system. If you live in the area, you get some points, if you have siblings in the school, more, if you child has a disability or health issue more still.....so bring along relevant papers for that as well.
Be assured you won't have everything you need and you will be sent back for something more, so go early. DO NOT WAIT TILL THE LAST DAY!!!!!
Smile and be patient.