Teachers were not told the amount of sleep students received when completing weekly performance reports,yet they rated the students who had received 8 hours or less as having the most trouble recalling all the material.

关键是Teachers were not told the amount of sleep students received

老师们不清楚学生每周完成学业所需要的睡眠（总量），然而，他们却定位，睡眠为8小时或更少的学生，不能呈现所有内容（材料）。
Teachers were not told the amount of sleep students received 中 students received 为定语从句，修饰前面的the amount of sleep。were not told 为被动语态，“不被告知”也就是“不清楚”的意思。