When the Principal entered the class, a student on the blackboard.
<u>Explanation</u>:-
The given sentence is in past continuous tense.
The past continuous tense, also referred to as the past progressive tense, is used to express actions at a particular moment in the past. The action started before that moment but has not finished at that moment.
It is composed of two parts : the past tense of the verb "to be" (was/were), and the base of the main verb "+ing".