Takuya: "He [Tomoki] must only be in third grade or so..."
Takuya: "I'm in fifth grade! I'm older than him.
Izumi: "So we are the same age? Come on... I thought you were younger than me."
Takuya: "I'll be 11 years old soon!"
Izumi: "On which month?"
Izumi: "So I'm older after all. My birthday is in May, so I'm three months older."
Takuya: "You [Tomoki] can't. You're only in third grade, aren't you?"
- Given that Tomoki doesn't refute this claim, we can assume he's in third grade.
Junpei: "Well, I'm in sixth grade!"
J.P.: "Professor? I'm no professor, I'm only in the seventh grade."