Glad you asked this question. Many people want to stay a long time and you need a special visa to do so. For stays of 3 months or longer you need to apply for a multiple entry non-immigrant Educational visa. You can do so at any Thai embassy or consulate near you. You will need a letter of invitation to the camp as well as the Rawai registration documents, all of which I can email to you.
Please email me at dcampillo@hotmail.com. I need approximate dates and your full name as it appears on your passport and I will email you the letter and documents to apply for you visa.