Great question, KMC. Welcome to EpicSki!
The truth is, the moment you have two or more students in a class, you have a "split." No two people are identical, with identical profiles, aptitudes, needs, desires, dreams, and thoughts. Dealing with splits so that everyone in the group has his or her individual needs met is arguably the greatest skill of a good instructor.
And every case is unique. Your hypothetical "sore thumb" standout student happens quite often, I'm afraid, but you have to deal with each instance individually. There are few rules. But here are a few general thoughts.
"The strong get stronger and the weak catch up." If you try to run your lesson according to this directive, you can at least minimize the problem, even if you can't eliminate it. Many splits start small, and grow larger and less manageable, as the instructor tries to keep the faster student(s) inspired, losing the slower ones in the process. Once someone gets frustrated, or loses confidence in himself, any weakness tends to become exaggerated. On the other hand, if you cater only to the slowest students, the faster ones get bored. How can you keep them all on board?
Success breeds success. Everyone likes to succeed at something, and every time you do, it increases your confidence. The key is not necessarily to go "slow" for the slowest students--which again would probably bore the faster ones--but to provide many, many opportunities for EVERYONE to succeed. Have a great bag of tricks, so that you can, for example, have your class do many repetitions of essentially the same thing, but with a different little "twist" each time. The fast ones stay enthusiastic, without being "held back" by the slower ones, which they would not if you just repeated the same old thing over and over, and the slow ones get more practice, and more opportunities to succeed too--which they need. Af few successes, along with the inevitable confidence boost that accompanies them, and quite often the "slower" ones suddenly flourish!
Sometimes you just have to find the key for the "slower" students. Sometimes it's just a different movement cue, an alternate description, a new demonstration, a different teaching style, or an appeal to a different learning style or sensory preference that will do the trick. Sometimes a student who is weak in one thing has a particular strength in another, and once you discover what that is, you can exploit it to help him/her succeed. Sometimes it's finding out another sport or experience that they're good at, and "transferring" the similarities to skiing.
Personally, I hate to hand off a "slow" student to a "special" instructor, even if the opportunity exists. I'll do it if I have to, in the best interest of the student, but I'll question myself as to how I might have avoided losing him/her in the first place. While many "slow" students appreciate the special attention they get when they are handed to another instructor, presumably in a private or very small group of other "drop outs," they still can't avoid getting the message that they NEEDED special help, that they are not as adept as other students, and so on. And that thought can become a self-fulfilling prophecy, hindering further develpment even with the private instruction. I hate to send--or reinforce--that message!
So I'll do my best to keep everyone together, inspired, enthusiastic, and learning as much as possible. It is not always possible, and sometimes compromises must be made. But it's a worthy effort!