[Your Address] [City]
Date: [Date]
Respected [Teacher's Name],
I am writing this letter simply to say what I could never quite say in class: thank you. Now that [occasion, e.g. the results are out / I am moving to a new school], I keep realising how much of my progress carries your fingerprints.
When I joined your class, [Subject] frightened me. You explained patiently, answered my endless doubts without once making me feel small, and stayed back after class the week before the examination. More than the subject, you taught me that effort matters more than talent, and that asking questions is a strength.
I scored [marks/grade] in [Subject] — a result I once thought impossible — and the first person I wanted to tell was you. Whatever I go on to become, a part of the credit will always belong to your classroom.
Thank you for being a teacher in the truest sense. I hope to make you proud.
With deep respect and gratitude, Your student, [Your Name] Class [Class]