This is VERY common with children who have ODD due to some kind of neurological reason. Often if they're acting up at school but not at home it's due to the setting--either they aren't ready to handle school, it's not the right setting and/or teacher for them, they can't cope with the stimulation, structure, etc. If you are seeing behaviors due to stimulation at home multiply that by input from 18 other kids and imagine what it could be like for him.
Many children do just the opposite and hold it together in the outside world and then fly their true colors at home. This is equally hard because they'll be little angels at school and then come totally unglued the moment they hit the van or house.
If this were my child I'd go in and observe. If things didn't get better I'd probably try another preschool with maybe a different approach before moving forward for evaluations.
