Hi A., I have a son the same age and have been dealing with the same as well. My ex left the home when he was 4 yrs old and it was really hard on him. He behaves at home for me but would act out at day care & now kindergarten. The day care wasn't so bad. He had the same teacher who is an older woman who knew the situation & he felt comfortable with her but kindergarten was a whole new ball game. He was constantly in trouble at school some of the things I agreed with and some I thought were petty, however it had to be addressed. I started a behavoiral chart at home and worked in things for school as well. If he obtained so many magnets by the end of the week, he got to do something fun like a play date or ice cream. Occasionally a toy but not often. If he acted up in school he would lose magnets, which made it harder and depending on what he got in trouble for, he would lose his toy room and only get flash cards to play with for the rest of the day. Needless to say he didn't like that.
His teacher (kindergarten) was young & this is her first year to teach so not alot of experience and to top it off it was a very large class of 28 kids, which is too many in my opinion. I did have some problems with the teacher as well with some statements made to my son so I ended up changing his class, which is the best thing I could have done. He was placed in a class with an older teacher with only 17 children in the class. His behavoir has been fantastic & he loves his new teacher & friends. He gets up every morning wanting to go to school because it's fun. I feel like he is getting alot more individual attention, which he needs at this time, & I have already noticed an improvement in his work & his attitude.
What you are going thru is just a bump in the road and you & your son will get over it in time. That is the key "Time". I wish you all the best & hope this helps.