This young instructor knows how to defoul plugs. I don't know any who don't know or don't have to do it on a fairly regular basis.
Since most schools frown on students or PPL's leaning the mixture, I don't know how many cracked cases are down to detonation rather than any other cause.
Whilst I will admit to some instructors not being stellar, the vast majority are competent and don't forget that once you have a licence, the duty falls on to you as PIC to be as knowledgeable as possible. The business of instruction isn't easy on occasion, time pressures can be very high and since most of the theoretical side becomes self study, then blaming FI's for people's lack of understanding is a bit harsh.
I have lost count the number of times students of mine have said "yes I understand." Only to demonstrate very clearly that they haven't the foggiest idea really. Be honest with us and then maybe we'll beat the knowledge gap.
Oh and when an instructor asks you to read certain chapters in the text books before the next lesson, it would be nice if you actually did!
(Rant off!)