Given the stated ages (16 and 17) of characters in this RPG, I've determined that if everyone is to be in the same class, then they'd be in 2nd year.
Those who turn 16 on or before April 1st are in 2nd year, alongside those who turned 16 on or after April 2nd, which meant that they would join the former in the latter year when they turn 17 sometime during that school year.
We're currently in "Spring" in this RPG, and let's say three weeks into the first term. If we establish that it is 2016 in-game, and that the school year began on April 5, then this "Friday" occurring in-game right now is April 22.
So therefore, 2nd years.
Akihito, Yamamoto, Katsu, Hantaru, Claud are all 1999 fellows who turned 17 sometime on or after April 2nd.
Everyone else was born in 2000 on or before April 1st.