You can counter-argue what negligible effect an 11th person will have on the class. One extra test to mark, one extra seat... not much, really.
You can but as the union and the government set the limits you won't win. It is a working condition of class sizes that has been fought for over the last 100+ years and teachers are determined not to have these hard-won conditions rolled back.
Effectively what would be asking is for the teacher is to give less attention to those who are in the class becuase that is what would have to happen. Teachers have other classes besides Year 12 and, from watching the Extension 2 teachers at my school mark that work it isn't as simply as '1 extra question' but hours of extra work for one extra student. The extension 2 eacher at my school had 4 exams to mark during the trials and it took him as long to mark those four papers with corrections as it did the general teacher to mark the 26 papers he had to do - an extra student would have added an extra hour and a half to the marking of that trial paper - which other class would lose out? a year 7 maybe or would he have had to be less thorough in the marking of the extension papers.
It isn't as simple as kids would like it to be - there are rules in place about class sizes etc and they aren't just regulations but some are actually part of the legislation regarding teachers working conditions. It isn't often what we would like to do either but often our hands are tied - we might like to say sure I will take 11 but, as one deputy said to me a couple of years ago, does that mean next year you will be prepared to take two extra periods a week, or do we ask everyone to work more than the award says you are to work?
You are looking at things from the point of view of a student and I understand that while I am trying to show you that there is also another side - that of the teachers and that they themselves have to answer to higher authorities and also to their colleagues who might be pressured into doing more - and then governments argue well class sizes can be larger etc.