1. 它们（根据上下文，这里指学生的作业）不是为了让学生与材料的深度相吻合而设计，而是为了易于分级以便和大量的学生相适应而设计的。

2. 现场课程并不常有，而且通常安排在中午，而这时学生还要去上其他课或工作。

这里when 引导定语从句，先行词为the middle of the day。

3. 老师的办公时间也可能是许多学生不方便的时间。

附：原文：

Online classes began to be popularized just a few decades ago. They are advertised as a way for adults to finish their education and students to learn the material at their own pace —it is far more compatible for people with busy schedules.

But after being enrolled in an online course last fall semester, I came to realize online classes were merely a means to fulfil course requirements. 

First of all, students lack the desire to learn, and they simply complete their assignments to receive credit for a passing grade rather than genuinely engage with the course material. 

As online courses tend to have more than 100 students, most of the assignments are short and simple. They are not designed for students to interact with the material in depth but designed to be graded easily to accommodate such a large number of students.

Perhaps the biggest disadvantage of taking an online class is the absence of face-to-face interaction between the teacher and their students. Live sessions are infrequent and are often scheduled during the middle of the day when students have to attend other classes or work. The office hours of the professor may also be during inconvenient times for many students as well. Most interaction with the professor has to be through email which is often impersonal. It is nearly impossible for students to build a relationship with their professor.

There is also little interaction amongst students. It can be harder for students to create study groups and form relationships with their peers. （以下原文省略）