If this note is for real, it shows education at its worst.
Dear Mrs [redacted],
You may alread know this, but in case Alex has neglected to tell you, I am assigning him to detention for one hour this Friday, April 22nd. The reason is as follows:
Alex consistently defied me. During class he contradicted me numerous times when I insisted that the length of one kilometer was greater than that of one mile. Every other student in class accepted my lesson without argument, but your son refused to believe what I told him, offering such rebuttals as, “You’re lying to the class,” and commanding other students to challenge my curriculum.
Although he was correct, Alex’s actions show a blatant disregard for authority, and a complete lack of respect for his school. In the future, Alex would be better off simply accepting my teachings without resistance.
Please see to it that your son understands this.
I would be thrilled about my kid, and appalled about the school, if I got this note sent home.