If the medical evaluation for the child is that there is nothing wrong, then behavior modification is the only thing to do.
The most effective behavior modification I have seen is to praise praise praise and reward reward reward when things go well. There are two books that talk about systematic behavior modification programs to be used -- one is How to Behave So Your Children Will Too and another is SOS Help for Parents.
One thing--- all of you mention children that sort of fall apart in groups. In other situations they are ok. It is possible that these groups cause a child a high level of anxiety and this can make them act out. Has anyone evaluated any of these children for anxiety?
This is similar to my situation. My five year old is terrible when he's with other kids. Our second sitter doesn't want him around after 3 p.m. when the younger ones get up from naps, the older ones get off the bus. He hits, says mean things, punches them, kicks them, just does anything mean and no explanation why. The pediatrician recommended a therapist, no help. We are on our second therapist. I can't take him anywhere because he causes so much trouble. No one will take him if we need to get away because he is so bad. He almost got kicked out of the 3 yr. old preschool, the principle has talked to him twice in K. If you have any questions for me, I'd love to help, if not and I find out some good information, I will pass it on. good luck.
OMG, I am going through the same thing with my 4 year old son right now. Tested for everything and he is fine. Behavioral problems they say. The difference I have is that my son is in day care all day right now, no school. He is fine at home, we have no real issues, but once he is in daycare, he fights a lot. I am so frustrated myself because I am at a loss on what to do. Nothing works for us either. This is his 3rd preschool, got kicked out of 2 schools prior due to his behavior at 3 years old, then brought in a nanny and that did't work either. I wish I had some advise for you, but I don't. I am curious to hear the responses to your message. Good luck, and I hope we can figure this out.