Well, i can see why people kinda confused. Middle schooler in manga usually presented with sailor fuku, blazer and stuff usually for high school.
Anyway, Kubota being 12 / 13 makes Kusanagi Sensei reaction in chapter 3 more understanable.
....Alright, probably this what would you expect from a country that transitioning from monarchy/just attain its republic.
I just hope Reiner didn't plan to be a president for life.
So this either lead to:
1. Knight Captain's Face turn Heel because his moment of weakness
2. Knight Captain seek some ancient forbidden technique and almost kill himself in the end
3. Like in 1, Enemy try to tempt Knight-Captain but after wavering he reassert his loyalty and love to kingdom.
I guess that what you will get if you acting all annoying and assholish in team work. When you become a liability, someone from your team will just decided cut you loose.