Willfull 6 year old boy, on my last straw

I had the same thing happen to me a couple years back with my bio son and the neighbor kid. The school is approx. 5 miles in the middle of nowhere. We live in a very rural area. Both boys received the notice from the bus driver the same day. I let them have it. I told both boys that if they ever got a notice again that they will walk home from school and I will drive right behind them. I don't care if it's a 100 degrees or pouring rain, you will walk. The mother of the other boy agreed. They only took the bus home from school, we drove them to school, because to school they were 1st pick up with approx. 1 hour on the bus before school and after school they were 1st drop off with only a few minutes on the bus. My son and the neighbor kid does not have any emotional issues so I was not dealing with that. We never had anymore issues on the bus. I don't make idle threats and both boys knew that.