You may consider avoiding a NAS device altogether.
And opting for a server.
The reasons for this are performance, and future longevity.
The price would be similar to a high end NAS box, however performance would be much better.
The good thing with NAS box's is that they are low power, however this comes at the cost of them having a slow cpu's and other components which can't cope with the traffic that the drives themselves can read/write at.
A simple low power server could offer space for many more HDD's in any raid setup you desire for future proofing.
It would be able to cope with many more users reading/writing/streaming from the device at the same time, and at higher speeds.
The down side is that Management would not be as friendly as a something like a QNAP or synology, as their web based control panels are pretty good to be fair.
I cant see a 2 or 4 or even a 8tb system lasting long with students uploading RAW DV footage. Especially with data redundancy required. The data rates and storage needed for media creation these days are demanding. Maybe for 1 or 2 people a NAS would be sufficient, however for a class, and class's in the next couple of years, I'd be looking for far more capable and expandable solution.
A NAS would be a slightly more effective stop gap than what you currently have, which will be simple to implement.
A file server will be more of a initial headache to setup, however once implemented the befits will be fruitful for the long term.