I am going to learn painting. The course will end tomorrow and I will also have to pay the tuition fee. How should I talk to the teacher and how should I express it in language?

First of all, thank the teacher for his care and cultivation during this period. Then you can express your gratitude freely. For example, you not only learned knowledge here, but also learned a lot that cannot be learned in textbooks. Things, etc. You can also say that this period of time is the most unforgettable for you. Just mention your gratitude to the teacher and feel free to use it.