As a fire safety inspecting officer with school experience and a Chair of Governors, I would recommend that you do not go down the route of key operated break glass points for a school.
Many of the main reasons are high lighted above and there has been sound advice given, especially as it could be considered as a fire safety management problem, more than a fire alarm hardware issue.
I would offer a couple of more comments:
Key operated break glass points for fire alarms are more in keeping with smaller premises, partial need for security, low number of occupants with known risks such as a premises in use for HTM 88. It is not for a large school risk with larger numbers, many types of use and therefore risks, at all sorts of times.
Althouh size and occupant numbers are not quoted there can be quite large numbers involved i.e. in excess of 500 pupils and 50 staff. Most schools these days have a tendency to be involved far more with a community use, "open all hours". Therefore at all times when persons are in the school would this approach be appropraite? e.g. parents evening, pta events, cermonial or awards functions, social functions, hiring out of school premises to public etc etc.
I assume that the all aspects will be covered in the fire risk assessment on occupation as there are a good few variable risks associated with a larger school, without the extra issues such as this.
Hope these comments help further.