Honestly probably Sasuke didn't want to go to school for awhile after the Uchiha massacre, He most likely stayed out of school for a while. Then re entered sometime later.
It's funny some people are confused and don't know why Sasuke didn't go to school for awhile after the massacre. It's really not that hard to figure out.
Maybe it's like the chunin exams? Twice a year. Makes sense. He starts a bit sooner than Sasuke, fails 1st, Sasuke isn't able to take the second exam, pass the third.
Well Kakashi graduated at 5 if I remember right... so you can definitely try the test sooner if you have what it takes. I'm more surprised that Sasuke didn't end up finishing even before that, despite it showing him at the academy regularly. Although to be fair he wasn't that great at the time of graduation compared to some others. A-student yes but not legendary...