The Japanese education system is comprised of six years of elementary school, three years of middle school, three years of high school, and four years of college. While this system still holds true in Haikyuu!!, the show added another element by ranking students by academic prowess or intelligence.
Their designated title of "first-year" or "second-year" indicates what year they are in at school, but their class number, as in "class 5" or "class 1," tells fans whether or not that character is prepping for college or learning the basics in the classroom. Haikyuu!! even featured an arc where Hinata, Kageyama, and a few other Karasuno players had to study harder to be granted permission to practice and play volleyball.
Shoyo Hinata
When Hinata received his test marks, he became discouraged because his teacher immediately mentioned she would be happy to help in his make-up class. He didn't think he was going to need to catch up.
The sweetest and most determined character is not always the most intelligent. Social studies wasn't Hinata's best subject and he had to pick up study habits from Yachi to pass his exam before being allowed to go to the Summer Training Camp.
Ryunosuke Tanaka
Despite Tanaka's drive to be better in volleyball and always encourage his teammates, he isn't the most diligent when it comes to his classwork. As a second-year in class 1, Tanaka is a few levels below his best friend, Nishinoya.
This is evident before the Training Camp Arc when the second years need some tutoring from Ennoshita.
Kotarou Bokuto
One of the least intelligent characters among all the players in Haikyuu!!. Bokuto is often viewed as the competitive, fun-loving, and sometimes dramatically emotional captain of Fukurodani.
He isn't the brightest, and even though fans don't get to see his school life outside of volleyball, when Nekoma captain, Kuroo, picks on him about his intelligence, Bokuto stands up for himself saying he got a 47 on his trigonometry test. Kuroo said what all the fans were thinking, "Uh ... congratulations?"
Tobio Kageyama
Kageyama is not quite as lost as Hinata when it comes to education, but his shortcomings are around learning English. His game intelligence is very high and he even recites different plays to Daichi from memory, and he realizes that if he can do that, he can pass English class.
When Hinata and Kageyama finally convinced Tsukishima and Yamaguchi to help them study, Kageyama even denies that he would need to use English.
Yu Nishinoya
Not one of the most intelligent characters in the Karasuno team, but still able to help, Nishinoya assists Yamaguchi, Tsuki, and Yachi when they help Hinata and Kageyama study.
He is ranked as a class 3 second-year, but despite his ability to help his teammates, he also still ran the possibility of not being able to go to Tokyo. Nishinoya studies with his fellow second-years and Ennoshita and thus is able to go to Tokyo on time, unlike Hinata and Kageyama.
Kenma Kozume
Nekoma's setter, Kenma, lands right in the middle between his most and least intelligent volleyball friends and teammates. His intelligence when it comes to volleyball truly shines through when he creates nearly every strategy Nekoma uses and he is often referred to as the "heart" and the "brain" of the team.
As of 2018, his role as a university student and with a job like a stock trader means his intelligence is high enough to make the fifth spot on the list.
Tsukishima & Yamaguchi
Kei Tsukishima and Tadashi Yamaguchi are both first-years in class 4. Tsukishima is blunt and can come across as arrogant, but he uses it to hide his insecurities. The only thing he doesn't seem to be insecure about is his intelligence. His game intelligence was already pretty high, but it increases after every training opportunity.
Yamaguchi is Tsukishima's best friend, but he is pretty timid. His intelligence is evident in how hard-working he is in and out of volleyball. His game intelligence was pretty low in the beginning, but over time, he grew to understand how his serves actually worked, increasing his understanding of the game.
Hitoka Yachi
As a first-year in class 5, Yachi's grades were exceptional and her notes were specific to her learning style. She liked to doodle in her notebook to help her understand or remember her lessons.
When Hinata asked for her help, she doodled a bit in his notebook as she was tutoring him, and was able to help him with his misunderstandings. Yachi is normally nervous and on edge, and while she was at first when he asked for her help, they soon fell into a routine and she was more confident in her teaching ability. Her game intelligence was zero until she started to focus on Hinata and Kageyama's quick attack and listen to Kiyoko's commentary.
Keiji Akaashi
Akaashi is the composed and encouraging vice-captain and setter for Fukurodani. He is a second-year in class 6. Like most setters, Akaashi is quick-witted when it comes to where to send sets and is analytical of the game of volleyball.
As of 2018, Akaashi is an editor for a major manga publisher. His skills in writing and reading led him to pursue a career in literature. His interpersonal intelligence of knowing when to leave his teammates alone versus helping them is also a major factor in his placement on this list.
Yuuji Terushima
While he's not in the show for long, Terushima from Johzenji is the smartest player in all of Haikyuu!! since he is a second-year in class 7.
Compared to the other players in the show, Terushima's class ranking may be the highest. His intelligence around the game of volleyball is also pretty high, but his enthusiasm can sometimes be his downfall, like when he tries to replicate the synchronized attack without ever having practiced it before.