5 year old misbehaving at school

My son is in Kindergarten and ever since he started I've been recieving phonecalls and notes sent home from his teacher about his behavior. He constantly tries to run away while at school, and he keeps throwing tantrums when he doesn't get his way (hiding under his desk, crying and screaming, saying bad words).

We have had issues with his behavior since last year, and I had mentioned it then to our family doctor who suggested an 8 week behavioral program which we completed, but nothing has changed.

I'm not sure what to do anymore. I've given him groundings, explained multiple times how running away is dangerous, and followed everything we learned during the behavioral program our doctor recommended and it seems like nothing is helping.

Does anyone have any advice? TIA.