Before the new school year the classroom was thoroughle ... up.

Before the new school year the classroom was thoroughly cleaned up.

До нового учебного года класс был кропотливо очищен.

The sentence is in Past Indefinite and the sentence is in Passive voice.

In Passive voice the subject takes the action. The action is more important than the doer in Passive Voice. The predicate of Passive voice is formed with the help of the auxiliary verb to be and we use the third form of the verb. In this sentence the predicate is was cleaned.

Before the new school year they cleaned up the classroom thoroughly. (Active Voice).