I fell in love with the doll on the left in this old Haute Doll advertisement from 2009. She is one of my dream dolls and is labeled as "SD16 School Head D." So I assume that the head is on a SD16 body too. Lately though, the way the neck looks oddly long compared to the rest of the body. Is this just me? Do all the necks of Volks bodies have that elongation, or is it just this one? Would that head even work on something smaller?
School heads are meant to work on most Super Dollfie bodies, but it might sometimes be that they look a bit too big on some (School A is quite a bobble head on SD10, for instance) or a bit small on some necks (School C and D are much smaller and can sometimes look a bit wonky on bigger necks). Here's a comparison I found in Flickr to illustrate what I mean. Sd16 necks are a bit longer than SD10 and SD13 girl bodies have so that might explain why it looks a bit funny. I think SDGr's neck is thinner, if you want a more mature body without having to modify the neck. School D comes up very rarely so best of luck hunting one down! (But I know the feeling keeping eyes out for a school C myself haha)
I feel like most BJD have long necks, but putting that petite School D head on the SD16 looks a little "perch-y" to me. But stuff like that varies from one owner to the next. Too perch-y to my eye doesn't mean it reads that way to everyone. I feel like School D is an even more delicate sculpt than School C. I had a School C that worked nicely on an SD13 boy body and that neck is similar to SD16 girl. If I was dead set on making a School D head work on an SD16 girl body, I would modify the neck - shorten and taper it a bit so the head fit properly. Otherwise I would also go with an SDGr if we're only looking at an all Volks doll. Volks tends to have small groups of dolls that are variations on one another and end up looking familial. There are a couple other Volks heads similar to School D that are slightly less petite, Madoka and Shizu. Madoka and Shizu are the same sculpt, I've read that the newer Shizus were perhaps retooled a bit to fit the SDGr girl body better? I don't know if that's the case or not. But both versions have been seen releases as SD16 girls If you're not planning to take the doll to a Volks event, you could consider a hybrid with a body that has a slimmer neck.
Honestly, if I had my pick I think I'd go for the SDGr body or maybe an SD13. I've always liked the clothes that are made for the SD and up sizes better than the ones available for SDMidi etc.