Question

The principal spotted Indira (1) / which was sitting in (2) / the canteen at the time when (3) /
she should be attending the classes. (4) No error (5)

Answer 1

Mistake is in 2 and 4 . In place of which there must be who.
In place of classes there must be class.

But nowadays classes (word) has become common . so correct answer is 2.