What happens if I cancel my student visa on the 16th in Thailand?

If you are on ED-plus extension, you are supposed to be able to apply for working (non-b, non-o working for organizations, or perhaps non-rs) without leaving kingdom. Why not talk with your local immigration office and your employer in future.

If you cancel your extension, you have to leave within the day as ED-plus cannot have future data to terminate the extension.

Are you graduating with a Degree from a university in Thailand ?

Or are you completing your course in a Thai language school ?

The answer is VERY important.

As you are eligible for a one year Ed Plus extension if you are graduating from a Thai university.

Another thing if you want to buy more time is to request and buy any extension which you won't be eligible for and you will get a 7-Day extension denied stamp giving you 7 days from the date of application to leave the country.

Once visa is cancelled you will have to leave the country before midnight of the day in which you go cancel your visa BUT you can ask your school/university to issue an official letter to show at the immigration stating that your visa -precisely your student status with that institution- will cease its validity on 20th. This will give you 4 days time (supposing you’ll go to IO on 16th) until 23:59 of the 20th to leave the country. Mods can tell you better, but I went through the same thing and that’s what I did.