to complain about teacher

[5plusMeAndHim]

DD is I'm y12 she is doing the standard 4 AS levels and has 2 teachers for each.In one subject she is getting solid A grades with one teacher and consistently getting 20 percent scores with the other.I do not think a gap this wide and this consistent can be down to anything except crap teaching.Cousins at school also think she is poor.we are in an affluent area and I think the problem is disguised a lot by tutoring

Don’t go in with a complaint, but ask for a meeting to discuss your daughter’s results. Clearly it needs to be investigated, but there’s likely a good explanation other than the teacher just being shit.

As an opposing point of view, I actually experience this at AS maths. Could not get on with one teachers way of teaching. He was a very new oxford graduate and unfortunately assumed everyone was as clever as him. If you didn't understand the way he explained it, he couldn't explain it any other way, and would just say the same thing louder and louder until I pretended I understood. Go a D in my AS paper. Second year I changed teachers to someone with a bit more experience and explained things in different ways until I got it.
I also got a tutor who went through thousands (not kidding) of past papers until I realised that with maths, each paper is essentially the same, with the same topics, just worded different, different numbers etc. Once I knew what to look for, and what numbers to put in what formulas it all clicked into place, even if I didn't necessarily understand why it needed to be done that way.
Ended up redoing all the exams from the first year and ended up with a B overall.

So the teacher may be making a difference, but it doesn't make that 'crap', I would ask for a meeting to try and explain the differences in grades. And I recommend a tutor.

She is 'generally' thought of as a bad teacher? How would you be in a position to say that?