Hi Misha, first, your father's photos:
#5 - not a cossack uniform, just a peasant-style shirt and a cossack-style astrakhan hat.
#9 - not a cadet, but a gymnasia or real high-school student.
Who is a military school cadet, it's the guy at #1 and #3. The photos are too small to say exactly which school, but #3 was certainly taken a year later than #1, because the guy is already sergeant cadet.