Hi,
you can assign a default value in registration via URL parameters, see example at http://docs.easy-profile.com/index.php/article/tips-and-tricks

for example:
- Create a menu item of type Users -> Registration Form for default registration
- now you can create another menu item of type System->External URL with same URL of previous menu item but with parameter to set value of your field.

for example:
default registration (Teachers): http://yourdomain.com/registration
registration for (Students): http://yourdomain.com/registration?role=student