The age of your children will be your main consideration when choosing a school. Younger pupils can benefit by immediately attending a local Spanish school, as this enables them to quickly grasp the language and integrate them into the Spanish way of life.
If your children are older you may want to consider the/English private schools. This would help make a smooth transition for life in Spain, whilst learning the Spanish language in an English speaking school.
As in the Uk, children are required to attend the state school located in the vicinity of their home which is determined by post code. Depending where you live you may or may not have a choice of schools.
All children in Spanish schools learn another language usually being English or French. In the Valencian region children are also obliged to learn Valenciano, the local dialect. This is very important to the people of this area and you'll find it's widely spoken along side Castellano, the national Spanish Language. What an advantage this is for children living here in comparison to those in the UK.
Sport activities play a large part of school life. Due to the excellent weather conditions the outdoor sports facilities are excellent and all schools participate in competitive sporting events.
As in the Uk some schools have a uniform and some not. Class sizes are up to 25 pupils maximum. Parent involvement is very important, Spanish schools are very sociable places and you'll find the attitude towards the parents very different from that in the Uk.
If you're worried about re-locating to Spain because of education concerns, Don't Be. It's probably one of the best choices you'll ever make for your children.