I can't speak for Tahitian dance because I've never studied it. You mentioned you'd like to "book" a class: have you ever studied Hawaiian dance before? And are you looking to attend a class or take a private lesson?

What I can tell you that I do know about Hawaiian dance is that it is unusual for someone to just drop in for one (or even two or three) lessons, unless you already have training from a particular teacher or have been recommended by your teacher to visit someone. Hula generally requires that you learn the basics of a school/teacher, and that can take anywhere from several months to several years. If you are a more advanced hula dancer, you might be able to "drop in" on a class as a visitor, but to do so, you should obtain direct permission from the teacher, or kumu, preferably at the recommendation of your own teacher. Kumu hula discourage what is known as "halau (school) hopping" because each school comes from a lineage of hula teachers, and there is a certain mystique or privacy related to each school's methodology.

If you are not an experienced hula dancer, what I would recommend is instead of trying to find someone to teach you (which might prove to be either difficult ... or very expensive), try to find as many good hula and/or Tahitian dance performances as you can to experience. There are hula-related events going on all year round on every island; the "Alternative Hawaii Vacation Guide" is a great online source for finding them.

One more piece of advice, if you don't mind: for only about two weeks of travel, I'd stick to only a couple of the islands. It might give you a little bit more of an opportunity to delve deeper into the rich cultural life there than can be experienced at the beaches and tourist spots.

I hope this has been helpful.