Hi Barry,
The reason why teacher, non-editing teacher and student aren't listed in the 'Assign system roles' page is because normally they are assigned at course level. However, if you want to assign them at system level, you can do so by going to Administration > Site administration > Users > Permissions > Define roles, editing the role and ticking the system checkbox for 'Context types where this role may be assigned'.
Note that if you assign a user the system role of teacher, then they will have teacher rights in every course on the site, including the front page.
Please see the documentation Assign roles for further information on contexts.