For Spanish school First register with town hall, obtain NIE number from local police. You will need to visit your local town hall to register the children for school, requirements may vary however, some things you will need.
Childrens passport, parents passport, childrens birth certificate, vacanation records, 2 photos, house deeds or rental contract. you will need original and copies of all these.
For private International Schools, not so much paper work.