Is it ok to say 'save them time'?

Question

227. Typically, professors instruct students to complete all background reading by the due date,

making the assumption that this will save them time during class.

Answer 1

The structure of this sentence is confusing.  Since there are two plural nouns, "professors" and "students", it is not clear to which group the word "them" refers.  The idiom "save them time" in general is not an excellent choice of words.  Classes are a set length, so time can't really be saved.  You might say that a professor's time is better spent engaging students in learning activities other than summarizing reading material.