Make a table that tracks which aircraft each instructor can train in and limit customers from selecting inapplicable instructors
as a for instance; if an instructor is not an MEI you could have a pop-up that tells the student that the instructor chosen is not certified to teach in the twin.
Currently, you can set an Activity Types to make sure that the customer and/or Instructor "meet the aircraft's PIC requirements". For example, if the Activity Type is checking the Instructor for this and the aircraft "is Tailwheel" then we would check this. Keep in mind, users would need updated Pilot Certificates in order to really start enforcing these (you could data enter yourself, or enable users access on their own). So, it is possible to enforce these.
However, I do see some scenarios where it can be simpler to just added an instructor and select which aircraft they can be scheduled for. This would be much easier to set up and can have the same net effect.
Rodney Hoover commented
Yes, please make this. Not every instructor can fly every plane at a flight school. There should be a way to limit students from booking flights that a particular instructor can't fly.