Hi

I would also have suggested open-xchange, although in a testing phase,
there were a few problems (cannot recall which ones).

I have a simple, but not optimal (and not security-oriented) "solution":
I carry my (encrypted, private) .pst-file on my memory stick with me.
From time to time (once a week) I make a backup of that file on a
storage server. This idea, of course, requires that every student
has its own memory stick, either provided from the school's budget,
or bought by him/herself. Hm, it also requires that your students
are allowed to mount a USB memory stick; compatibility with your
policies etc. I assume that no 200 GB (or so) dedicated server
storage is available. .pst-files can be positioned wherever you want
them - there is no need to store them locally - a network share is
fine. But keep in mind that network bandwidth (I have no clue) might
be an issue.

With the USB memory stick, speed is not an issue - my .pst-file is
about 500 MB in size.

Cheers