4 year old not listening

I have 2 children in the same pre-school. My step-daughter is 4 1/2 and my son will be 4 in about a month. For the last 3 weeks (since school started) his pre-school teachers have reported regularly that my son is refusing to listen to the teachers. There are several occasion they have reported the same thing about my step-daughter. At one point they said that he is screaming at the teachers about not doing things, throwing toys, screming when he is sent to time out, threating to spit on them, and even one time hit a teacher who asked him to sit down. My daughters behavior has improved slightly, although has been better in the past (as well as my son's). The kids have been going to the same school regularly since April and had issues when they first got there, but improved greatly since, until recently. There teachers are concerned about how we handle not listening at home. The clincher is, we don't have this issue at home. They listen very attentively to me and there father. They rarely fight, and never hit or threaten us. We were at one point, taking toys away for bad behavior in school, grounding, time out, and extensive talks about there behavior. We are at wits ends about what to do now. We feel like we are punishing our children for nothing, because they are good kids seemingly every where BUT school. Their reports are still the same at school, and I am feeling like the teachers are frustrated with us because, to them, were not doing our part as parents to stop the behavior. Is there anything we can do to try and curve the school behavior? I am fearful that when they go to kindergarden things will just get worse. But how do I fix something I can't see? Is this just a phase for testing the school waters? Please help

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I have a 4 y + 8 months turning 5 in October. He is really a great child, friendly, full of energy, healthy. BUT, has a huge problem listening to instruction.  I dont know if its just the age or if he has an underlying behavioural problem.  I have tried everything - reasoning, begging, bribing, talking calmly, being firm, but if he says he wont then he wont.  It also seems to be like a little meltdown if I can call it that.  He totally loses it and this happens mostly in public places. For example - yesterday after Kindergarten we had to go to the Optometrist to fit his new glasses.  I had told him before Kindergarten in the morning and he agreed to go without a fight.  After Kindergarten in front of all teachers and other parents. Kyle wanted to walk home with his friends and not go in the car with me, he walked in the opposite direction, crying and performing! I calmly picked up my daughter and walked to Kyle kneeled before him explaining that we came to Kindergarten with the car and would have to go to the Optometrist before he could play with his friends. If he behaved and listened then I would consider him playing in the afternoon.  It took me about 15 min to get him to the car. My other concern is that my 21 month old daughter then also starts to cry and hit me or her brother as my son also hits/pushes me when he has these meltdowns.  I am not sure what to do anymore - I spoke to a friend yesterday and she told me her son & daughter also behave like this. I know I am not alone but what can we as parents do better to handle and resolve these situations at the time?? Please help!