I think the thing the girls learned was that boys are gross

some day they have to learn it ..... sure boys are gross .... we are a strange species

True and we, as a group, need others to deal with our various stinks and general grossness

Awwwwwwwawwwwsw cheerile looks so cute in this, and blushy big mac is best mac. Also the girls are taking the lesson differently, and that adds to there already cute characters.
(thanks for doing the idea, I love the outcome, and just how cute this picture is. Another idea that would be cute is dash, or another pony, wearing layers and layers of diapers, so much so that they can't even stand on there own two feet.)

Also could you come up with some explanation as to why some adult ponies are dressed and treated like foals?

uffff.... what explanation could that be?

Maybe a illness has spread across ponyville, mentally regressing ponies, and causing them to rely on there family and friends.

It goes on for so long that clothes and other items used for coals are made for the regressed adults, and as time goes on it become more natural for everypony.

Is the one getting changed Big Macintosh?

yeah ..... maybe he should be more muscular