> can't understand how algebra works and is getting only 1s and 2s, it's too bad, he'll have to repeat the whole grade again I guess.

That hardcore math was only in schools that specialized in math. In post-Soviet times, if you had a good engineering university nearby, the juniors and seniors were heavily encouraged to attend additional evening math classes there as well. But if you were in one of the "humanities" schools, the requirements were way more relaxed and they would not make you repeat the grade, they would just give you a 3-.