Hi,
It would be a good idea to stress test the system before taking it into use. Or, if you cannot do that well enough, try to add users in smaller batches. If you need to support all the 200 in one step, then atleast have ready the availability to add CPU power/memory, when needed.
I would guess, that teachers have a very on/off usage of the system. During lessons, they are not using emails, and during breaks, everyone is suddenly reading and sending emails. If the system is not fast enough, then you will have problems with the new users complaining about the system. Which can be much more expensive than the hardware.
You might also need to do backups at the same time than there is load on the system. Check the usability then and in other scenarios.
Rgs,
Friskis