No way they're 16. By the way they act, I'd guess they are 14 years old tops. I'm not an expert when it comes to the Japanese school system, but I know that middle school is three years long, and high school is three years long (it's 4 years in America, 9th-12th grade). If they're in their second year of middle school, that means they're 13 or 14. Middle school third years would be 14-15. (Feel free to correct me if I'm wrong)
I don't remember the manga specifically saying they were in middle school, but even with the moe artstyle, they sure don't look like highschoolers to me.