It used to be that you had to go to a classroom, and there would be some diligent teacher -- some genius teacher in there -- but that person was only in there at certain times and you had to go then, and then was not most times.
Antes, tínhamos que  ir a uma sala de aula, onde estaria um  professor diligente — um professor genial — mas ele apenas estaria lá a certas horas e teríamos de lá ir, e não seria muitas vezes.