Why do you look for private lesson ?

First of all, I worry about being misplaced. Once I had an interview and I was completely misplaced. Also I think that one-to-one classes you have all the time for your self; you do not have to share with other slower, weaker students. Listening too much wrong English, you start copying and teachers lack time to correct to everybody appropriately. I also think that the student should always choose the teacher they want to have lesson with.
That's why I'd rather have private lesson - in large groups you do not have a chance to ask and solve your doubts.