When does the 90-day reporting start for my Teacher visa in Thailand?

The 90-day reporting requirement does not begin until you are on a visa or extension that is valid for more than 90 days. Since you are currently on a tourist visa which is valid only until June 13 and awaiting a Teacher visa, your 90-day reporting will start after you receive your Teacher visa, which is typically a one-year visa. When you obtain your new visa, the immigration office will provide you with a slip detailing when your 90-day report is due.