Just how old are the characters? Like, they're high school age, right? And second years about... So they're only like 14-16 years old. And the leaders of the tank teams are at most probably like 18 or something.
They have to be old enough to drive... or at least drive tanks.
But I think they are of high school age. Whatever the average age of freshman are the ages of the freshman group, or the same with Sophomores, Juniors, and Seniors.