- Is this the last exam of you guys?
- Is this the last exam for you guys?
Could you explain which sentence is correct?
They're both grammatical. The first is also called a physical examination with the guys as patients - in this case, the final in a series of check-ups. The second could refer to the same, but is more commonly understood as the final test taken by the guys in a course of study.
Using of carries implications that something of the guys is being examined, like eyes.
Using for carries the implication that what is being tested is separate from them themselves.
So if you are talking to a group of Army recruits being processed into basic training, then of may be correct, but a group of college students during exam week, for is correct.