For those who don't know how assessment works in Art - the kids complete their work in supervised conditions, usually on a couple of dropdown days, where they miss the other lessons and stay in Art all day.
The teachers (usually two) look at the work and mark it, then send the marks off to the exam board. Sometimes they will enlist help from art teachers in other schools, especially if the teachers are fairly new or there's been a change in exam board.
The moderators are booked either before or after that point (depending on how late they want to push it). On the day of the moderators' visit, the work belonging to the students they have selected for checking is made available to them - they usually pick the highest marks, the lowest marks and some in the middle. Often the teachers will also have a couple of other fairly certain graded works from other students to hand, just in case. If the moderators agree with the marks the class teachers have given for each piece in their sample, the marks for the entire cohort are accepted. If they disagree, they can (and do) regrade every single student's work unseen, whether higher or lower.
Composition and Performances in Music (and Devised Pieces, vivas for Technical Strand students and the final performances for Drama) are marked in a similar manner, as are Music Technology. If there is a very small class, every student has their work moderated in a visit. In addition, video and/or music files as appropriate for every student are sent to the board with the assessed grades and moderators are paid to decide whether the performances in the sample group match the grade specifications.
Essentially, if the class teachers get it wrong on one or two pieces, they can find the entire cohort's grades rise or fall with no comeback because the samples showed they assessed wrongly. So it's in their interests to get the 'right' mark and to encourage, cajole, persuade and, in some case, outright demand that they have input into the work, because they want students to get the highest grade they are capable of.
This is a fairly simplified description, it's a lot more stressful and detailed than that, but it does boil down to this Art Teacher is the one who marks her final work. So the kids need to listen to her, even if it annoys them to do so.