My mom graduated from Poly in 1952. I have a high school daughter who plays basketball and we were looking at the old yearbooks at my mom's house -- and we noticed the weight classes. I have never seen it anywhere else and I made a weak attempt at Google research.
I also asked my husband's uber-sporty relatives in Michigan and they were not familiar with ranking kids by weight, so maybe it was localized?
And a big P.S. How many high school basketball players are now pushing 200 lbs? We are just heavier people than we were back in the 50's.