Haha, no worries ;-)
Ko Tao is never really quiet in my experience, there are always plenty of people about even in so-called low season. But the dive schools will be hungry for business and you should be able to negotiate yourself a good deal. See if you can get a discount for the course, and definitely make sure you get free accommodation for at least 4 nights if you're doing a dive course.
Last time I was there, traveling with 2 others, we got free accommodation for 5 nights in Coral Grand resort, even though only one of us was diving with the onsite dive school Coral Grand Divers (recommended). Always ask for a discount or a better deal, especially as it's low season.
Probably, you'll be approached on the boat before you even set foot on the island, by dive school reps trying to sign you up for a course. Personally I would prefer to wander around and check out several dive schools, seeing which one you prefer, before committing to a course.