I agree with 'Crashem'.
A gift is not something you should feel that you have to give or something that should be expected.
Presenting your instructor with a small token of your appreaciation is a nice thing to do and shows class.

You're not "buying" the service by giving a small present, it's just your way of telling "hey, thanks man, appreaciated that you took time too teach me".

It's kind of a company giving a christmas gift to it's employees. It's not needed, it's not called for, but it's a nice gesture of respect.