A schoolkid's cheeky logic. The first speaker uses the conditional 遅刻したら + だめ ('if you're late, that's no good') — a standard softer imperative. The reply picks up the same conditional but inverts the logic: if the teacher is also late, it's ノーカン. ノーカン is an English-derived slang contraction of 'no count' — meaning 'doesn't count, off the record, doesn't apply.' ですよね invites rhetorical agreement. The humor is in the asymmetric proposal disguised as fairness.