Sorry to say I’m not 100% certain since they never mention it in the anime and I haven’t read the novel or seen any of the live actions, but they do mention the characters are in junior high. Most junior high schools in Japan tend to encompass 7th, 8th, and 9th grade. The characters already know their homeroom teacher and say they had him last year so they probably aren’t in 7th grade. Some sources online say they’re in 8th grade which would make sense since the characters aren’t worrying much about getting into high school, and if they’re in 8th grade the characters would be 13 - 14 years old. Hope that helps!
- Mod M