You are making an argument that people do so, do you have any evidence for this ?

There is a negligible difference in glycemic index / glycemic load between the variations of M.F. milk products. Some analysis has skim milk as having a lower GI.

Unflavoured Milk is not relevant to the GI conversation.

Ok, there's no different.

Beyond that, Minor differents in glycemic load are irrelevant if you're consuming milk with a meal, like the kids in school are doing.