Step 1: Find out about the master clock you are replacing
Thus the clocks in the school all showed about the same time, but all wrong, and always wrong. This proves that the expression "even a broken clock is right two times a day" is false.
You will need to know:
* what protocol is used by the slave clocks (can probably guess based on the make of the clocks)
* how many zones are used for bells (indoor, outdoor, different buildings etc)
Your school (or other location) may even have documentation in the form of wiring diagrams. These can be very helpful when installing the new clock.