First - what bloke said. Period.
King souzys need a particular neck and two dissimilar bits combination. If you don't have the right order to the bits into the right neck, it won't line up, you can't adjust it to the face of the player, and intonation will be off.
That said, I have always been a proponent of a rebuilt souzy. I put my money where my mouthpiece is: I play a reconditioned Bundy/Selmer Signet with a Jupiter neck and bits, with a blokepiece Imperial with a 32.6 Helleberg lexan rim and a spacer trimmed down to .080. It's roughly the same geometry as a Conn 7B, but with a slightly wider cup and a different backbore profile. The upper first valve loop has been converted to a slide so I can "ride throttle" on the usual suspects. I have played just about every souzy made, ancient and modern, since football season in 1976, and owned several along the way.
That said, if you must purchase new, I would get the new version of the Conn 36K, which is the same horn, same factory, same line, same mechanicals, and same everything else except for the sticker on the bell and probably using two of the same bits instead of dissimilar bits as the King. I don't know what mouthpiece comes with them, but whatever it is will be fine on the field for students. Hopefully it is something like a 7B, so it is big enough to get a good tone, but not too big to handle for students, especially if you have any cross-over students, like I was, from trumpet, bone or euph. If the bits are the King dissimilar bits, then it doesn't matter whether you get the King or the Conn; get the best deal. It is EXACTLY the same horn.
(The old version of the 36K was the fiberglass version of the 14K, and if you were to get refurbs, my preference would be for either the 14K's or the old 36K's - many out there, many shops have a whole morgue full of "crash parts," and they have overall good tone, projection, and are rather light for their respective families - brass or fiberglass.)
Yamaha is just too dammed expensive for what they are, and you will have valve problems with the valve guides breaking. Period.
As far as posture and positioning, I suggest you have your students consult the Marine Corp Band manual:
http://www.marines.mil/Portals/59/MCO%205000_18.PDF" target="_blank" target="_blank
And not just for the souzy players, but for all players.
The neck and bits have to come to the face around from the "outside," with the receiver end of the neck pointed roughly to the right, from the player's left, and the receiver and two bits making rather an arc to the face, so there is maximum adjustability to the player's stature and embouchure. Too many times, the neck is pointed the other way, and it impinges on everything, resulting in the contortions and not wearing the souzy properly, causing undue fatigue, strain, and bad playing.
Finally, do consider refurbs of older horns. They will cost about half of new, and if refurbed properly, will last as long, if not longer. Anything that is prone to breaking from bad design or execution will have been fixed.