Very naughty child ***HELP***

Alright mommas! I need help! My 3rd child was born 3 weeks ago and since then my 6 year old has been TERRIBLE! Both at home and at school. At home he's disrespectful, rude, disobedient, mouthy, etc... he throws tantrums and refuses to do as he's asked. We've tried talking to him and letting him know that we still love him and care about him despite the new baby. At school he is awful! He's a very mean child! And he'll completely shut down if he gets in trouble. He refuses to participate in class and won't do his work when he is asked to. The picture shows his behaviors in class over the last 3 days! We've grounded him from all his electronics.. he doesn't seem to care.

I don't know how much more of this I can take. I try to spend time with him but I'm a single mom during the week and with a newborn it's very hard. It's also winter where we live so doing activities outside is out of the question. Anyone have any tips or advice???