Answer:
Dear Father, 
I understand that you have received a report from the principal of my school. I understand that the principal has complained of my stubbornness. I understand that you have threatened to deal ruthlessly with me if my actions affect the respect that people have for you. 
I want to assure you that I take my responsibilities seriously and I understand the importance of respecting the opinions of others. I understand that I have been stubborn on some occasions, however I can assure you that I am trying my best to improve and to make sure my actions do not affect the respect that people have for you. 
I hope you can understand my position and trust that I am doing my best to improve. 
Sincerely, 
[Your Name]
Step-by-step explanation: