After the first day of the 3 year old class, I'm certain that the 4 year old class would be a better fit. This has a little to do with the academics. The school is not a highly academic preschool, but they do begin to introduce writing letters and numbers in prek and this is something he's wanting to do at home.
More importantly, I think it is a better fit socially and for play. He knows how to do school - sit for circle time, raise his hand, stand in line, etc. He has always prefered older friends. The toys and books in the prek class are the type of things he enjoys at home, while the toys in the preschool class are what he played with about 2 years ago. During the time for learning stations (when the kids choose what to do), the options in prek are much more interesting to him.
When DS is bored, he has behavioral problems. When he is excited about something, he's so much fun and generally so respectful. He was bored on the first day of school this time - very different from last year where preschool opened up a whole new world and he came home so excited. I want to give him the opportunity to be in a place where he looks around the classroom and is so excited.
It seems like an easy choice for me, at least for this year. I don't know what the plan will be for next year - like some of you, I don't know where he'll be in a year. We live in an area where we have lots of options. I wouldn't want to skip K, but I am open to early entrance in part because our local district is very open to it if a child tests in.
