I am currently living in Bangkok and I love astronomy. However, I haven't been able to find an official astronomy exam that I can do to gain college credit. Could someone help me out please?
There is a Dantes Standardized Subjet Test (DSST) in astronomy. I have no idea if and how widely colleges give credit for these exams. Typically, you will start with physics and choose to specialize in astronomy. Therefore, most standardized exams are for physics which may have a handful of astronomy questions.
I know this is an old question, but if someone is still wondering ... your country regularly sends teams to the International Olympiad on Astronomy and Astrophysics. I am also certain there are Thai teams at the International Astronomy Olympiad as well as other International Astronomy Olympiads. I know there are people who "discovered" astronomy competitions and then got involved with them. Although they are not standardized tests per se, I think astronomy competitions/Olympiads ought to be mentioned.
There is a GCSE in astronomy offered by Edexcel: https://qualifications.pearson.com/en/qualifications/edexcel-gcses/astronomy-2017.html
The assessment has two parts, an exam, and a series of two observation task (aided and unaided).
The course is suitable for 14-16-year-olds.