Dear teacher:
Hello!
First of all, I want to apologize to you. Due to my negligence, I accidentally kicked you in the process of playing football. I'm sorry, I hope the teacher will forgive me. I know the teacher asked me to write a critical letter for my own good, and let me remember this lesson in case the consequences will be serious next time. I promise you, I won't be careless and cause harm to others in the future.
XXX？