Twix - we are ALL paying for our children's education. Some people just choose to pay twice. Thank you.
Be aware that private does not necessarily equate to a better education. Teachers do not have to be qualified and do not have to follow what is currently felt to be best practice. I am a teacher in a private school, btw.
Provided the teacher is well-versed and prepared, all will be fine. If s/he is not up to speed, then it is likely that whether the class is mixed or not will be the least of your worries.
A good teacher will always be differentiating activities and grouping children according to their strengths and weaknesses in the different areas of the curriculum, and making adjustments throughout the year.
However, if a teacher is assuming that single age = same work for all, your child will be a lot worse off than a child in a mixed class. Sadly there are independent schools that work on the basis that if the child doesn't fit their idea of what they should be doing, the child is the problem. State schools are better equipped to deal with whatever/whoever is thrown at them, because they have to.
My DD is currently in a mixed YearR/Year 1 class, and will go into a mixed Year1/year 2 class. It's what small schools do, and I think that it often works extremely well. She plays as much with Year 1s as with her fellow year Rs, and has been inspired by the more tricky stuff that they have been doing. Next year she will be able to look to the Year 2s. A summer born boy, for example, (the group least likely to cope well with our early school starting laws) will find the pressure less.