Dear {Teacher},
I never thought I'd say this, but I actually don't want the semester to end. Your classes have been a joy to attendâI'm so glad I took them!
I really had fun learning about {subject}, especially {favorite topics}. If someone had told me before that I'd be a {subject} geek by the end of the semester, I would have laughed at them.
Thank you for sharing your knowledge and opening my eyes to the wonderful world of {subject/topic}. I will miss your {passion for teaching/kindness/etc.}.
Warm regards,
{Your Name}