Same thing: shorter, harder songs changed into a more singable key. For the most part, the school editions aren't to make the shows accessible to high school audiences, they're done to make some shows easier for high schools to perform. Les Mis, Ragtime, and Sweeney are all incredibly difficult shows to do, and many high schools wish, but never had the resources/talent, to do them. With shorter scripts, and songs transposed to easier keys, high schools won't feel as intimidated by these pieces and will thus produce them.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.