After reading your question just now, I can see that you are a good student with deep feelings and gratitude. If your teacher sees you leave for him/her, he/she will be moved even if you don't say anything. ...

There are many ways to express gratitude, such as; Say thank you, hug at parting, sincere wishes, greeting cards, farewell party at parting, and a card signed by you collectively.

Now that technology is so advanced, we can still leave each other's phone numbers. If we have a good relationship, we can keep in touch often in the future. You can also meet or something if you have time. You can do anything.

You know, parting is not reunion after a long separation.