Is there any difference between roles Teacher and  TA?

The difference between the roles of Teacher and TA is almost entirely cosmetic.
The only real difference is that a TA cannot assign, or override, or switch to, role of Teacher. 

A user  who is listed in the Shnaton is automatically assigned  a role as follows:
  • Teacher
    • If he teaches any lesson-type other than תרגיל
    • If the only lesson-type in the course is תרגיל
  • TA
    • If there is more than one lesson-type in the course and he teaches only lessons of type תרגיל