Statement of purpose: One-for-all or one-to-one? This question is about statement of purpose (s.o.p).

I am not sure that whether it is advisable to write exactly one s.o.p. for all the schools or to write for each school exactly one s.o.p.? Does this really matter? Would the reviewers of a school judge applicants by their understanding of the school? 

I really need judicious suggestions.
I assume you are talking about a document that is part of your application to universities (e.g. for a PhD), which would be called a research statement here in the UK. Then the answer is simple: you want to maximise your chances of each application. So invest the extra effort and make individual documents to maximise your chances.

Different departments have different priorities, so you want to emphasise how your plans fit to the research agenda of each place you are applying to. While it may be convenient to send the same SOP to each place, you can show that you have researched the place you are applying to and at least modify the SOP to fit to the place you are applying to.