Don't spread misinformation. There is no mandatory school uniform in Russia. Girls dress in a similar outfit like in this model for graduation, because it's kinda cute.

In most universities around the world, a bachelor's headdress and a robe are not mandatory for students. This nonsense has been preserved only in some universities in England and in a couple of other former colonial countries.<Sarcasm off>

Some correction to your 'correction'. First of all, the original Soviet uniform was completely different, with a dress at least twice as long. There is no nationwide uniform in Russia, let alone a Soviet-style one. These are modern, unofficial cosplay outfits worn by girls strictly for their graduation ceremonies. This tradition stems from the fact that schools didn't have any uniform for around 15 years at all; at most, there were loose dress code guidelines for some elite schools, but not actual uniforms, and the girls actually liked the late-Soviet aprons. These outfits often contradict real school dress codes—if a school even has one, as rules are set by each individual school now. This graduation tradition, as well as decentralized school regulations and preferred school uniform designs, is common across many post-Soviet states, not just Russia and Belarus. Next time, try doing some basic research before confidently sharing your misconceptions about other countries you know nothing about.