Dear [Teacher],
I regret to inform you that I will not be able to attend the scheduled class as I had to take a medical appointment related to a prior injury. 
I understand that this is not ideal and apologize for any inconvenience caused. I have already taken the necessary steps to keep up with the course curriculum and will keep you updated on my progress.

Once again, I apologize for the sudden change and I assure you that I will complete the coursework diligently. 

Thank you for your understanding.

Sincerely,
[Your Name]
Motivo: devi prendere un appuntamento medico.
Componi un'e-mail di 2 paragrafi a un insegnante spiegando perché non sei in grado di frequentare la lezione programmata.