Question

As soon as the teacher entered the class, we all (stand)up

Answer 1

Hey mare!!

No sooner did the teacher enter the class, than we all stood up.

Hope it will help you@_@

Answer 2

The answer to your query is:

' As soon as the teacher entered the class, we all stood up.'

The 'stand up' will change into 'stood up' here because the sentence referred to in the question is in past and is narrating an incident that has already occurred.

While referring to sentences in past, we use either 2nd form of the verb or the 3rd form of the been and here also we have done the same.