You should never punish your child when he is not doing well in school. Better try to narrow down the issue with following steps:

Step 1: Try to understand from teacher why he is not doing well. Try to know specific reason from teacher.

Step 2: Once you know the the reason then discuss with your spouse about the different ways to help the child. It could be special tuition for specific subject and personal level coaching etc.

Step 3: Spend time with child to play his/her favorite game together. Give your personal and quality time to the child. Make friendship with your child. Don’t show him or act like you are parent or superior than him. Bring your self down to the level of your kid then he will open up with you.

Personally, I would recommend to somehow convince him to do Mantra meditation for 10–15 mins before going into school. Sitting quietly and chanting Syllable OM and doing Pranayama, also taking deep breath. This will help him to become focused and increased concentration power.

Thank you !!