I was always in a composite class in primary school as there were only about 80-100 students in the school. It was nice. We had K/1 , 1/2 , 3/4 , and 4/5/6.

I remember in year 6, my friends brother who was in year 4 was put in the 3/4 class, so he wouldnt be in the 4/5/6 with his sister. I think their mum may have requested that, as his name was on our class roll but he was in the other class. You can definately request it and the school should try their very best to do what you ask. It shouldnt be that hard. Its not as if you are asking them to rearrange all of the classes in the whole school. Maybe they just wnt to see how they both go in the same class and if there are problems, they might reconsider your request?