For your case,

1. Ask your school for a letter to cancel your NON - ED at Prachuap Immigration

2. You can apply for a 30 day extension with the letter from Embassy of Vietnam at Prachuap Immigration

3. Or you also can apply for this one at Chaeng Wattana. But you must have TM30 in Bangkok

I really appreciate your help

How much will I be charged for the processing or without any payment?

How much do I be charged for this processing?

I did extend my visa to 6 months

It's also the ED visa

Non-Immigrant Visa "ED" (Education)

This type of visa is issued to applicants who wish to study, attend seminar, training session, or internship in Thailand.

If it's possible, you can apply for an extension of stay at the Office of the Immigration Bureau and may be granted such extension for a period of one year from the date of first entry into Thailand

I see. Thank you :)

I got it. Thank you so much for your comment