Mapping it out makes answering this easy, but is there any cohesive way to do this under strict time constraints?

Missed one question.

I got the two answers correct fairly easily, but I need to move much quicker than I did to answer them due to the time constraints of the LSAT.

This question is written poorly. "Each student who received a grade lower than a B- missed at least one class session." We (the readers) do not have information on the grades of the people who attended every class. While reading conventionally, to assume all-attending students got a B- or higher is not appropriate without information, such as stating "ONLY students who missed at least one class session received a grade lower than a B-".