Does one work in or on an aeroplane?
Both are correct. However, I would say the student's answer is more apt.
Terminology for airplanes is similar to ships (they are in effect airships).
One works on a ship. Passengers on a flight manifest are quaintly referred to as souls, as they are on a ship. The pilot is a captain. The co-captain is also the first officer. Stewardesses are so named because of ship's stewards before them.
If you would say a captain works on a ship, then a flight attendant works on an airplane.