Hi L.,
Ooooh, I think this is getting out of hand. Punishment administered hours after the offense is too disconnected for such a young child. Of course parents must be informed and supportive of the teacher, but surely there is some time-out process at school that delivers the consequences right after the act. if the teacher expects you to punish at home for behavior at school, it is the teacher who is at fault. She should be watching your daughter more closely if this is a recurring problem and certainly should be taking responsibility for what happens in her classroom. Furthermore, sitting in her room till dinner is not only too harsh, but not something a 3-yr-old even understands. What is she supposed to do? Think about what a bad girl she is? And when is dinner anyway, given that she can't see you making it and can't tell time?
With all this, of course you need a more positive way to deal with your poor little girl, who is trying to be good -- otherwise she wouldn't still be in school! -- but doesn't have control of her temper yet. I taught my boys with pretty good success to "think in your mind!!!!" when they felt themselves getting angry, then to walk or run away from what's making them angry, and if necessary to strike out at something else, never a person. i also urged them to tell me about anything that made them angry anywhere other than home, and talked frequently with teachers about how they were doing in school and at home. I'm guessing your daughter feels displaced by the baby but also loves the baby and has enough self-control not to hurt her. But she's got some anger that pops out in other situations, among her peers and perhaps with friends too.
It may sound dismissive, but I think if you are patient and can get the teacher invested w/a lot of discussion, you will get your daughter past this. Just please don't confine her to her room -- it makes her feel bad, alone and terrible, and you my find her reluctant to go to bed one of these days, or indeed holing herself up in there when she's older, if her room is where she is sent when things are bad. Please talk about this with her rather than just punishing. She can be "helping" with dinner while you talk about how hard it is to handle anger. Try it and good luck!
S. F.
mom to two strapping grown-up guys!