When I first read your confession, I thought there might be some school uniform variants I had overlooked or that the topic needed deeper research.
But since you mentioned “it's not that hard to Google,” I’ve simply gone with what shows up from that search.
Belarus "Last Bell" graduation dress
Hetalia Belarus’s purple dress:
_Graduation dress: Simple, straight-cut dress with a plain white apron. No puff sleeves or frills.
_Hetalia: Puffy sleeves, heavy frills, decorative bows, and layered skirt. They are all common in Victorian or Lolita fashion. The large ribbon belt is also a typical Victorian/Lolita design element
Graduation dress: White, an apron that covers from chest, maid-like in shape.
Hetalia: Also white, but the apron starts from the waist down and is more like layered like a simple Lolita overskirt.
If the similarity you're referring to is just the white apron, then I'm sorry I have to disagree. This element alone isn’t unique to Soviet or Last Bell uniforms. White aprons are widely used in maid, housewife, Victorian, Lolita, and other retro styles.
You could argue that Hima has stylised the dress (like other characters military uniforms) but even then it would just be some minor twisted.
In Belarus's case, the overlap between the graduation dress and Hetalia Belarus’s outfit seems to come down to popular, widely used elements like a white apron, or white shocks and black shoes which also appear in other fashion styles (Victorian, maid, Lolita, etc.). That feels more like normal coincidence than deliberate uniform accuracy.
HOWEVER, if you were actually referring to another variant of the school uniform that's genuinely closer to Belarus’s canon purple dress, I’m open to seeing it.
Feel free to send the link via anon ask on my Tumblr or through another Hetalia hot take, either way works.
And if I did get the information wrong, I'd like to apologize.