Flight school airplanes unfortunately are subject to all kinds of neglect at the hands of students and instructors alike.
Reef knot is used when the tension will be on the long ends, and you want to be able to spill the knot by tugging on a short end. This doesn't sound like an appropriate application.
A permanent tie-down, what could be sadder? Get out and fly that thing!
Sorry for the delay following up, I got sidetracked reading a large totally unrelated thread. This forum is dangerous.
Jason