7. When the teacher (came / was coming) into the classroom, the children (sat / were sitting) at their desks.

7. When the teacher came into the classroom, the children were sitting at their desks.

В данном предложении необходимо использовать Past Simple для действия, которое завершилось (the teacher came into the classroom), и Past Continuous для действия, которое происходило в определенный момент в прошлом (the children were sitting at their desks).

Ответ: came / were sitting