Here is how you do that: Advertise in the beginning that there will be X number of horn players chosen from that class at semester. Then promote that every day. Let the kids know that everyone is eligible, but the top level players are all that will be considered for the change over. Then play audio of cool horn parts as the kids are getting ready for class.
That lets you find the smart students who will work at it and gives you a hint at who the musicians are. It's a little early to know a lot about that, but you can get some insight by semester.
Then when you move the students to horn, let them keep playing out of the trumpet book. They will sound a fifth lower, but they will get used to that very quickly. (If you move them to the horn book, they are either playing too high or too low for a young player to be successful very quickly.)
If you decide to start horns and trumpets together, do the same thing with the book thing. That is, start the horns in the trumpet book. The fingerings are very similar, and it is good ear training for those horn players. The entire class will get a faster start and move along much more quickly than if they started in the beginning horn book. But the fastest start is teaching a trumpet class for one semester.
You can decide when to switch them to the horn book based on their progress and embouchure strength and flexibility.
All the best for a great start to the year.