as of now groups are used as a way dividing people in different interest areas for easy subscriptions of different topics of interest.
I would like to use groups as a way of giving, say a teacher access to only his/her class of students in the catalogue. Also vice versa, I would like the students to see only their teacher and not the teachers of other courses that is not relevant to them.
You can go to Front End PM PRO > Settings > Recipient and uncheck “Can users add themself to group.”. So you can assign group members yourself and students can send message to that group only. If teacher also a member of that group, he will also receive that message.
But if you want one-to-one message (student can send message to his teacher privately), also teacher can only access to his students that need to be custom coded.