It is a good reason to switch, but there are other considerations as well. If the private schools are growing, you can imagine how many will be in public school classes. Also, the academic program matters a great deal, as does your involvement in whatever school he is in. Look at the teachers as well. If they are super teachers, they will still be super with 20 in the class versus 9. 20 is not a huge class number anymore either. Is one of the school's advertising points the small class size? If so, is the tuition going to go down if the class size they touted in years past is doubled? Cost is a valid consideration as well. If after considering other factors, is the cost still worth what you know your child is receiving or is it a deal breaker? If the main reason you have him in this school is the class size and personal attention, then your reasons for wanting to switch are completely valid as you won't be paying the high cost for getting the same thing the public schools offer. Lots to consider!