One way to say goodbye to a student in Engpsh is to simply say "Goodbye" or "See you later". If you want to be more formal, you can say "Farewell" or "Take care". If you want to be more informal and friendly, you could say "Bye" or "Later". Here are some examples:

Goodbye, students. Have a great day.
Farewell, students. It was nice teaching you.
Bye, students. See you tomorrow.
Later, students. Enjoy the rest of your day.

Overall, the choice of phrase depends on the context and your relationship with the students. It is important to use a phrase that is appropriate and respectful.