What is the process to extend an ED Visa in Thailand after applying for a single entry at the consulate?

Now if on one of those 90 day extensions you wanted to leave the country and come back you would need to BUY a re-entry permit (a single re-entry is 1000baht and a multiple re-entry is 3800baht) <- BUT a re-entry permit is ONLY valid for the length of time you're stamped in to thailand for. Meaning in your case being on 90 day extensions a re-entry permit is only good for 90 days. Then it would expire, so if you're NOT traveling during that 90 days DON'T buy a re-entry permit.

Okay, you're confusing two completely different things

Non-Immigrant VISAS sold outside the country are sold as a 90 day single entry OR a year-long, multiple entry.

IF the consulate you're using will only sell you a 90 day single entry, when you arrive here you're stamped in for 90 days and your visa is done.

When that 90 days is running out, you take documentation from the school and go to the immigration office and apply for a 90 day extension for 1900baht. You do this every 3 months for the duration of your course..