No sooner did the teacher (1)/ enter the class room (2)/ the students got up. (3)/ No error. (4)

Answer

2

Explanation

than will be used after the class room because - No sooner than indicates that the second event occurs immediately after the first.
no sooner introduces the event that occurs first.
Look at the examples given below:
No sooner did I come to the office, than the phone rang.
No sooner had he gone out, than he came in.