I am sorry, but you still keep saying the same things like in the german forum. As I said, there is no webservice to get the roles of a user. But if you'd tell us what do you want to do, maybe there is an alternative way to achieve your goal.
If you want to get all users from course for example you maybe can use the webservice function: "core_enrol_get_enrolled_users_with_capability". By naming the capability you could differ between students and teachers.
The problem with roles is that they can be changed without to inform the external system. Capabilities for example always are the same.