You don't have a choice ahaha.

The classes were predetermind by the GMs, infact Sho put them in a post before.
Also Nobara is a second year and all the first year characters are in the sane class anyway :p
Which includes Chiaki

Edit:

1 - 1 (1-2 to 1-3 also exist, but not many first years)
Midori
Isamu
Chiaki

2 - 1
Rui
Noboru
Sachiko
Leiko

2 - 2
Teiji
Akane
Yakudatsu

2 - 3
N/A

3 - 1
Aiko and Akari
Ayano

3 - 2
Jace
Kami
Akemi
Kato

3 - 3
Sato
Alexei
Matthew