Here is the original item:
Once class was over,
(A) Maria
used to meet her lab partner Chris at the diner near
campus (B)
until she discovered that Chris
preferred,
(C) to flirt
with the waitresses instead of discussing the chemistry homework.
You wanted to do this:
Once class was over Maria used to meet her
lab partner Chris at the diner near campus until she discovered that Chris
preferred, to flirt with the waitresses instead
of discussing the chemistry homework.
You need a comma after over. Once class was over is a subordinate clause, and when you begin a sentence with such a clause, you must use a comma to attach it.
You might want to review the rules.