The teacher told the class that all the lights must \_\_\_\_ before they leave the classroom.

must 為情態動詞，其後須接原形動詞；主詞 all the lights 為動作的承受者，須使用被動語態「be + 過去分詞」。switch off 的過去分詞為 switched off，故 be switched off 完全符合語法與句意。