Which sentence is correct? They or them?

"It was them who cleaned the classroom."

or

"It was they who cleaned the classroom?"

Answer

Grammatically speaking, the second one (It is they) is correct. According to Grammar Girl:

... when a pronoun follows a linking verb, such as "is," the pronoun should be in the subject case.

Later on, she elaborates on the subject by saying:

Linking verbs are words like "is," "was," "were," "appear," and "seem," which don't describe an action so much as describe a state of being. When pronouns follow these non-action verbs, you use the subject pronouns such as "I," "she," "he," "they," and "we."

She also gives us a few examples:

Who called Jodie? It was he.

Who told you about it? It was I.

Who had the phone conversation? It must have been they.

Who cares? It is we.