I plan to enroll there after 11, but I haven't quite figured out how the studies work there. Do I need to "complete" 12th grade in the dzumbi kyouiku katei course? Is it considered a separate 12th grade, or will the study of Japanese be alongside it, or how? Do I need to study the dzumbi course first and then go to the general course?
I would recommend, in that case, to study for a year at a Japanese university, programs like A-level. American ones definitely close after 1 year. As an alternative, study for a year at a university or college in Russia.
There are additional subjects in such programs at the university.
Look for schools that ideally have at least partial government participation, so there are Japanese as founders. And check how many years the school has been established. I don't know anything about this one.
Also, pay attention to where the school is located. A year in a large interesting city versus a village. Well, Tokyo is cool, but housing will be expensive or very far away.