In my day (Gawd!) back in the sixties and seventies anyone who became an instructor before he became a proper pilot was known as "scummed off" rather than creamed off. This system had been in operation since WW1 when my instructor on Tiger Moths (Flying Scholarship) only survived the war thanks to being kept back as an instructor on Maurice Farman "Pushers".
The main requirement in selecting instructors should have been not in selecting from the brightest and the best but in selecting from those who had the ability to impart skills and knowledge and above all, had the desire and interest to do so.
The courses at CFS were simply filled by Air Secs branch from any who were available. No checks were made of their psychological aptitude or even their interest in the subject.
Not suprisingly this resulted in many instructors who would have preferred to have been anywhere but instructing... and having to tolerate some young sprog sitting in front of or next to them.
Some did their best, others behaved as demented screaming school girls who did little to improve their students abilities or motivation.
Some of the WW2 pilots remarked how their contacts with the enemy were always far more enjoyable than their contacts with their QFI's had been!
I'm sure that things are much better in today's Air Force and that only those who want to instruct, scummed off or not, get to be a QFI. (!)