Classes just ended which free's up Jen's time.
I'll start by commenting that the comments about the need for a comma before "which" and the elimination of the apostrophe were accurate.
As for the verb tense...I completely understand the argument that "classes just ended" and "frees up Jen's time" do not agree in tense. I think that the key here, though, is the information that is
implied in the sentence...and implied portions of sentences
do affect their structure. An example would be "Bob can't run as fast as I." "I" is used, rather than "me" because of the implied word: "as fast as I (do)."
"Classes just ended" is being referenced in our controversial sentence as a fact that currently exists (present tense), allowing Jen's time to be free (also present tense).
Applying the same argument to a variant on the earlier comment about Lothar: "Mozart died far too young, which makes me mad." Certainly, you can not argue that Mozart's act of dying
made me mad at the time; I wasn't around to react directly to the event. Rather, it is the currently-existing
fact of his early demise that gets my goat in the here-and-now.
The original sentence, then, with implied information interjected, would be
"(It is a fact that) classes just ended. (This is a fact that) frees up Jen's time."
-PG