Thai immigrants go to school

It seems that you don't need a 90-day certificate letter, just fill out the form. However, new students who apply for a non-immigrant visa in China need the school's certificate for the first 90 days, and change it to a student visa with a validity period of 1 year, which is not needed in the future.

The full name of 90 days is "notice of staying in the kingdom for more than 90 days", which only means that you have stayed for more than 90 days, and reporting once means that you are still in Thailand and have not smuggled out.