The main problem is the decommissioning fee is variable depending on the type of connection the school has i.e. a Radio connection would probably cost twice that of a DSL one as it would require a 2 man team to take it down.
The issue from the schools perspective is they had no choice on how they were connected and at no point were warned that their connection type would incur costs £x.
At the end of the day i would never imagine OCL would sue schools (imagine the headlines in LEP!).
If it was me i would just refuse to pay, all other ISP's would build the decom costs into the original connection fee.