15. The new students in the classroom for 15 minutes before they realized that it was the wrong classroom. (wait)

15. The new students had been waiting in the classroom for 15 minutes before they realized that it was the wrong classroom.

Перевод: Новые студенты ждали в классе 15 минут, прежде чем поняли, что это не тот класс.

Ответ: had been waiting