What's the downside of going to 1st in the afternoon? The possibility of it stopping if they have another 1st grader they need room for is the biggest downside. Any others you know? Is he physically ready for all day/every day school?
What's the upside of going to 1st in the afternoon? He'll start associating with kids who will be in 2nd this fall, where you want him to be if I understand you correctly. He'll be introduced to different scheduling, teaching methods which may be different than K, but more like he'll encounter in 2nd.
Do you fear his current teacher will sabotage his success of K/1st split? Her comment about her experience disturbed me, as Dottie said, "tell her to check her baggage at the door".
