Because everything than happened between 4x22 and 5x06 happened in a month or a month and a half.
The characthers said several times, like Diane saying it’s been three weeks since Alicia stopped decoridng. Or the countdown for Cary and Alicia to leave it was 4 days, but 3 episodes took place in those days.
So it was still may or july and Zach was still in high school.
Then they made the time jump in the episode called “The Next Month” and another time jump to catch up with the real world and the holydays. So Zach is college now, they just haven’t show us.