Considering that your roaming profile storage location is big enough to house all of your user's 'stuff' (including their sync'd DropBoxes), you could switch from roaming profile to redirected Documents. However, you'd be trading latency on login/logout for latency while using the system, which may be worse.
Roaming profiles are typically used when users migrate from one workstation to another, instead of using the same system all the time. Redirected folders works well when everyone uses an assigned system, but one with little storage space OR a pressing need for off-system backup (such as student laptops that have to be reimaged a lot due to them downloading malware and/or treating the laptops badly enough to have to swap out drives).
My suggestion would be to enforce a GPO that pushes Dropbox folders to a Local folder on the student system. It wouldn't be redirected to a server, nor would it be sync'd on the network. In fact, syncing a DropBox folder anywhere is going to result in DROPBOX resyncing to their cloud at the same time, which might be part of your problem. Just let Dropbox sync locally, so as to not take up more network bandwidth than it already uses.
If you need to work with roaming profiles, set up exclusions on MP3 and video files as well - they tend to be large files that take time to resync. Instead enforce a policy that only allows them to be written to locally attached drives such as memory sticks or external hard drives. They should never be saved to My Documents, either redirected or roaming.