As for entering a graduation date/dates...
I do not know the exact wording on the Form 146; does it ask for expected/estimated graduation date for college?
You can always enter "not applicable".
Another option would be, on the down side that you are not accepted into the Academy for class of 2024, I would guess you would attend college, possibly one of the two from which you are taking dual credit courses. If so, how many semesters would you have left to finish your desired degree counting the college classes you have taken or will take before you graduate high school?
For example, let's say you'll have 20 college credit hours, then for most degrees you would need about 7 more semesters to complete a four-year degree. So, you could put an estimated/expected graduation date of December 2027.