Hi calgaryx.
You cannot hide the Sieve settings/the map tab.
Do the admins create a separate account for each student?
I would try a group account. Then the user logs in for e-mail (account) with their EGroupware access data. This means that a user would only have to change their EGroupware password. NOT the email account.
In principle, the problem would not arise if you authenticate the mail server and EGroupware against a shared user database. That would be normal…
Or use EGroupware-Mail-Server 
Stefan