Hi!
This is a very tricky question to answer just based on the information you have provided. Aruba representatives generally won´t say client numbers on an AP because it´s so much based on what those clients are and what kind of service they need.
If you want an educated guess I´d go for 325 or 335 if you want that much students (perhaps with multiple client devices each?) using latency sensitive applications. Make sure you work with an aruba partner that can assist you in proper RF design.
Cheers,