I agree that it could be tiredness, do they let them have a rest in the afternoon? She's dealing with all these new things, full days at school, social life, learning things, independance and yet she probably misses you lots. My dd still cries at school sometimes because she misses me.
Tell them at school what she is like at home, just in case there is another explanation. But after school can you not spend perhaps 20 mins with her going through her day and just spending one2one with her? She hasn't seen you all day and is probably jealous of your ds spending all this time with you. Then when she does get to see you, you have other priorities to deal with too, you cannot give her that personal attention that she needs.
It's a confusing time, but the things they relate to at this age are rules, because it helps them to feel secure. She is just testing those rules atm because she feels less secure, but stand firm and she'll soon blend back into family life!