If you're looking to study on the islands you most likely will be paying 20,000 baht to switch from a tourist visa to an ed visa. I live on Samui and just went through this process. I also spoke with a few different schools on the island and found that it is the same.
I am in Samui, had a stamp that said something about a 90 day report, so I ask when I went in Wednesday and she said no need to worry about it. But as mentioned above every IO is different and can vary!