You've described the quintessential chicken or egg problem. If people didn't f up, then there would be no need for the "victim" to respond negatively. Conversely, if "victims" were more tolerant, the perpetrator would possibly be more likely to fess up to any failings they may have.
To me...
Sounds like you should be prepared to settle for £40-£50 then, if you're intent on pursuing the school for compensation. Some on here think that you should; others that you shouldn't. Plus ca change...
I think it depends on what they said when they offered you £25. Critically, did they accept liability (or were silent, which would probably indicate acceptance of liability) or did they explicitly exclude liability (e.g. by saying the offer was an ex-gratia or goodwill gesture)?
If the former...