it is pretty common practice here. I think it works ok if your child is in the bottom grade and the split is fairly even (it always is here if possible). My son is a young year 2 (1st grade) in a year 1/2 class. He is ahead of his group in maths but there is no higher group and he is starting to get bored. Also I found as a child that really half the class was working from the year x text and the other half was working from the year y text without any interaction or cross over.
basically I don't think it makes any difference whether you are in a mixed class or not unless you have a flexible committed teacher. In which case you would be fine anyway.
