I think it takes *A LOT* of energy to adapt to a new situation like school, and clearly, DS was exhausted from holding it together during class (preschool).
I seriously did not appreciate how much WORK it is for kids to be in school all day. And first grade can be rough anyway, because there are generally much higher behavioral expectations than in K. Usually, K is still "fun". First grade is a lot more seat work, a lot more handwriting and a lot higher behavior expectations. And if it is also their first time going all day, that's a lot of transitions.
I know that when Sarah started first it was very hard on her. It took her well over 6 months to really adjust to the all day schedule. We had to cut out all after school activities, have a snack as soon as she got home, and she needed an hour or more to decompress.
So I think what you are experiencing is very normal. Not that the behavior is acceptable, but I think that reframing the situation to developing coping mechanisms vs seeing it as a situation to punish helps a great deal.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)