Exam

In the first exam, he had gotten a score second only to the record holder,
Namikaze Minato, and he had completed the second exam in the shortest time ever on record, even though he was alone, and not in the usual three-person cell. In the third exam, his opponent in the first match had defaulted, leaving him the win, while his performance in the second bout led officials to decide there was no need for him to take part in the third match. And so, his exams ended.

Obviously, Itachi was promoted to chunin.
Fugaku’s son was a ninja blessed with a terrifying talent. Occasionally,
Fugaku almost forgot the boy was his son, and felt jealous of the genius he
possessed.
“If a ninja of Itachi’s caliber joined the Military Police Force, it might perhaps improve the standing of the Uchihas within the village,” the long-haired Inabi said joyfully.

Book of Bright Light Chapter 3 Part 2