...like your CRM, Class Detail pages and Membership pages.

The quickest way to do this is to give them Front Desk permissions via Settings > Roles > Manage Staff > Edit a staff's role.

However, if you'd like to alter their teacher role to have access to this messaging capability, back up to the Staff Account Roles page and edit the teacher role. (Note that this will give all teachers the same permission changes you make here).

Next, under Home, you'll find 'Inbox: View and write to all conversations'. Check this box and Save.

Teachers can now message customers from your Inbox, your CRM, Class Detail pages and Membership pages.