Sudden behavior change...

Amanda • Trying for baby #3!
This is long so bare with me...All of a sudden our 3 year old is lashing out at day care. He is an extremely bright three year who tested at a cognitive level of a five year old and has been an angel at home. We have experienced no issues beyond the normal occasional 3 year old situations. 
BUT we have gotten notes from school 5 days in a row expressing concerns over his aggression. When he is getting frustrated he is slapping and spitting on the first kid he sees and has now started doing it to the teachers. They are concerned and have run out of ideas on how to control him. I am not sure what suggestions to give them as we are not having ANY of these issues at home. 
I stepped back to really analyze the situation to make sure we are not being blind to it at home and I am confident we are not. We are firm believers in working with our son to better control himself not to blame the teachers. 
Our life hasn't change any over the last week but we are expecting our second baby in December. I don't know if this is impacting anything or not. Please if anyone has any suggestions I am all ears.