I think your attitude is all wrong.
I've had children go down in my classes because they've had colleagues in previous years who've told them they're above target in y8 whilst handing in work that would fall short of current y6 standards.
Equally, I've had children dip in my class because in spring term we did Shakespeare and they found it difficult, but by July on a class novel they were flying again. Learning isn't linear. Students don't master everything at a set speed.
Regarding teaching, yes poor teaching can be an issue. I've seen it, coached weak teachers and have been the colleague who gets given a y11 group who've had a terrible y10 to get them back where they should be. It does happen.
However, what also happens is a teacher who is perhaps less effective overall (but not rubbish) struggles to teach a class because the students don't work, they opt out, blame the teacher, are deliberately awkward as a class (so the teacher struggles to teach the ones who do want to learn).
A whole class can drop due to poor teaching. A whole class can drop due to really nasty pack mentality of actively doing all they can to prevent the teacher teaching (usually with excuses like 'we only asked questions because we didn't understand' when the reality is interrupting every 5 seconds saying they don't get it which prevents the teacher teaching). They go home, get their parents angry, parents call school complaining, students know they are backed so become more rude.
I've seen all of these situations by the way.
I don't honestly believe at the moment you know enough to make a judgement either way and I've had meetings with parents and very sheepish students after acquiring these classes that highlights the students are often not the faultless victims.
To move forward, if you want her catching up then a tutor is probably the most effective way to get progress tightened up. Depending on who DC gets next year I would keep a close eye on her maths, and then speak to the teacher at the first occasion you have any concerns.