Depends on whether you want to do a proper job or not.
If so you will either need a specialist firm or someone very good with a JCB. Best method is to use tons of hardcore and membranes to get drainage sorted out.
Worth bearing in mind that there are a lot of schools out there that were put together on a budget and subsequently are too dry or wet and generally have a poor surface.
As I understand it you do need planning permission, something to do with changing the ground structure, i.e. digging and drainage. Best to check with your Local Authority planning department.
thanks for all the tips -we have just battled thru all the change of use malarky so I know we will need pp-we were allowed to build stables but had to get ppto be allowed to graze horses-beats me what they thought the stables were for