@FancyBird
I am not going to go over the whole cohort moderation but will add that they would also need to get permission off all of the pupils/students as well its a massive ball ache.
A few other things to consider that could have gone wrong/on.
The teacher was leaving and could just have said "fuck it", put general notes in and hoped for the best. Possible. Internal moderation and all that
It was marked by a substitute that didn't know what was going on and did their best. more likely, even with internal moderation as those internal moderating may not understand the mark scheme either.
The teacher marked the work and the school was forcing them to increase the marks at moderation. The teacher has left. So, could be.
It was marked by the new teacher, internal hire, didn't like the old teacher and done for malice. Unlikely.but possible
I have seen all of these especially where coursework is the main source of mark,
Also it is possible that the school (SLT) knows that its messed up and is trying to hide it.
The school will have a moderators report
It is also possible that if there is an exam that the teacher or sub could have taught the the wrong areas, but that would only mean that your DC's paper would need remarking.