Thanks, Emma, for your reply. The thing is I can only manipulate with the "enrol/self:unenrolself" capability when trying through admin/users/permissions/define roles/teacher. This is the only attribute that can be changed under Self-enrolment. It looks to me it is not possible to disable the self-enrollment permission just for certain roles. It is either enabled for everyone (who accepts the fact he/she will work as a student) or unavailable.
Hi
Work around
1) Create a special Role under USERS-->Define Role that has same privileges of Teacher and exclude the self enrollment permission.
2) Assign Role to user who is acted as teacher.
In this way original Teacher Role will be untouched .