This depends on the course. If it is an owner/operator then a small gift is acceptable (assuming you are happy with the class).
I taught at a US Sailing school for two years and tipping was very common from my students. I would say my average tip for a four day course was anywhere from $50-$100 depending. There were also the occasional party that did not tip, thats ok too.
It, like anything, should depend on how happy you are with the class, boat, teacher, and information delivered.
As a post-starving sailing instructor, I know those tips really made the difference and allowed me to continue that lifestyle.
Hope this helps!