This sounds a lot like my son. In kindergarten, he was always getting in trouble. He would lie down in class or under the table. He would get upset about different activities. When he got upset he would yell or shove chairs (with no intention of hurting another student but also absolutely not paying attention to whether or not another student would get hurt). It was horrible.
The teacher met wit the principal and with us and talked about how to help him. She tried all kinds of things and even asked for ideas from the principal and special ed and resource teachers. It helped, but it was still rough. She had a behavioral specialist come in and observe and received some great advice. Still, progress was very slow.
Over the summer before 1st grade I took him to speech (for social skills) and occupational therapy. We even saw a counselor. I met with his first grade teacher and told her everything I could. She also met with his kindergarten teacher. I mean, we were as prepared as we could be for 1st grade.
Every morning when I drove him to school, I said, "I love you! Listen to your teachers! Make good choices!" I tried to be as positive as I could.
I'm honestly about to cry as I type this. Our school worked so hard! The teachers tried everything they could think of. They listened to the experts. They worked with behavioral specialists. I just wasn't enough. Our son needed something different!
Just before Christmas (in 1st grade), the person in charge of special ed called us and told us about an alternative school our son could go to. BEST THING EVER!!! My son needed help. He needed someone who knew how to help him, and he was clearly (after 1 1/2 years of sincerely trying everything we could think of) not going to get that help at our neighborhood school.
Our son is now 10 years old and in 4th grade. He is thriving!!! He has since be diagnosed as being on the Autism Spectrum (which has allowed us more access to service - insurance wise), but being in the right school has the key to helping him.
Your son is only 7 years old. If he's having these types of behavior challenges, it's incredibly unlikely that it's just him choosing to not behave. There is almost certainly more going on. It could be that he has ADHD or sensory issues. Maybe he has some type of learning disability. Possibly an emotional issue. Please don't take any of this to me, "Oh my gosh, there's something WRONG with your son." I'm only saying, there's most likely an underlying cause, and the best thing you can do for him and try to figure out what that is and how to help him deal with that.
This is not about what you are doing at home. What you are doing at home is going to have very little affect on his behavior at school. This needs to be addressed at school. Meet with his teacher (again, if you have already). Say, "This isn't working. What else can we try?" But really, you're asking, "What else can you (the school) try?"
Ask to have him evaluated for special ed. Don't fear the stigmas. Seeking answers for your son is too important. Talk to your pediatrician. See what evaluations your pediatrician can refer you to. It's time to get to the bottom of this and find out what is having such a huge effect on your son. He shouldn't have to live like this. He shouldn't be so upset at school that he resorts to this type of behavior. Help him.