I went to a public school ( a long time ago!).Our whole class...

I went to a public school ( a long time ago!).
Our whole class (except myself and one other boy who went on to go to Oxford) failed the English O Level as the teacher had only taught us 1 book, when we would be examined on two books. I sussed this out and got the study notes for a book on the curriculum, Wuthering Heights, and passed with an A.
Our whole maths AO class failed. The bright kids did their maths O level a year early, and spent the following year doing the next grade up, an AO exam. Five different teachers for five different classes per week. I swear a couple of them had mental issues. We weren't taught anything. I told my Mum I would fail through no fault of my own, and with her blessing I rang in sick and was 'absent' on exam day as I did not want a 'fail' on my record. Everyone else failed.
So, yes, this can happen, and does happen.
Best idea is to make sure you know what is in the curriculum, and either get your kid to self study, or get a tutor if you are worried about the standard of the teacher.
And definitely complain re this particular teacher. Sometime head teachers are desperate to sack crap teachers but they cannot unless a number of complaints are made.