"The Talmud (Temura 14b) records that at a certain point in history, the Rabbis felt that there was a serious concern that the insistence on observing this point of law could possibly cause much of the Oral Law to be forgotten, so they permitted the transmitting of the Torah Shebe’al Peh from a written text." (source)

"The third grade gets three hours Chumash (Five Books of Moses) and one hour Torah she-be'al peh." (Glinert)