Sure, here’s a short and grammatically correct letter in English that fits within the 50-70 word range:
Dear Mr. Smith,
I hope this message finds you well. I am writing to inform you that I will not be able to attend class tomorrow due to a medical appointment. I will make sure to catch up on any missed work and would appreciate any materials or assignments given during my absence.
Thank you for your understanding.
Best regards,
[Your Name]
This letter follows the standard format of a formal letter: it begins with a polite greeting, provides a clear and concise reason for writing, and ends with a courteous closing.