When writing a Mother's Day letter in English for a high school level, you can express your gratitude, love, and appreciation for all the things your mother has done for you. Here is a sample letter:

Dear Mom,

You are not just my mom; you are my best friend, my mentor, and my inspiration. Thank you for teaching me the importance of hard work, kindness, and resilience. Your love is the foundation upon which I build my dreams, and I am eternally grateful for the sacrifices you have made for me.

On this special day, I want to let you know how much I love and appreciate you. You are the definition of strength and beauty, and I am so lucky to call you my mom.

[Your Name]