If the school has a Homegroup set up then you would need the school's Homegroup password in order to access any files on other Homegroup machines in the schools network.
If this machine was in your own network's Homegroup then it's required that you leave that Homegroup before you can join another one.
You also can't change a Homegroup password, a new password is created each time you create a new Homegroup but this password can't be changed.