If you would like to prevent users from scheduling instructors for a lesson in an aircraft that the instructor is not checked out in, you can do the following:
1. Enter checkouts for each instructor in the instructor edit screen (Resource Info >> Instructors). See grid in the image below. Instructor checkouts can be entered by model.
2. Go to System Admin >> Group Options to find the check box for enforcing instructor checkouts. (See image below. Check box is highlighted in red.) Checking the box will block general users from booking a lesson with an instructor that is not checked out in the aircraft they are scheduling - only if the two are booked at the same time.