No, there is no functionality like this in .LRN / OpenACS at the moment, *but* you can easily add this by copying the code from /acs-admin/users/one ("Login as this user") and make a link to this functionality in /dotlrn/clubs/<clubname>/members, if the user is an administrator of this club (and you can replace club with class).
There is one *huge* drawback to this approach. The professor can not only see all the data the student has entered into the site (and look what the student has been talking about in other clubs), but also post as this user. And I'm pretty sure you don't want to enable this, unless you inform all your students that in the system professors are able to take on students' identities :).
Instead I'd provide each professor with a student login (using a bogus email address) as a quick fix.
