Play Safe - Put the Mast on the School
The radiation from a phone held up to the ear is always going to be many times greater than anybody will get from a mast, unless they climb it and put their ear in contact with the antenna. DECT and similar cordless phones have substantially higher outputs than most mobiles so not using mobiles does not protect you if you use cordless handsets...
Most masts and handsets use adaptive power control so if they are close to each other and getting good reception they both reduce power. Thus the safest place to have the mast from a child's point of view is on top of the school building.
Most of the mast signal will pass overhead and whats left will be attenuated by the building's structure
The child's own mobile will operate at minimum power reducing their exposure to the minimum (provided they are on the network using that mast of course!)
The mast itself will be at minimum power when the kids in the schoolyard talk to it.
The school gets income from the mast provider.
The UK could do with listening a lot less to the NIMBY's before they manage to turn the place into an open air museum, they have turned planning processes into farce not only on mobile sites but anything that looks like a useful bit of contruction. It sounds like the US is going the same way.