Can I apply for an ED Visa during a 30-Day TR Extension in Thailand?

and on that note I think the O/P knows to TALK to the school they are interested in attending and see what they say.

Just did it in Bangkok. The whole procedure was much faster than expected (I was the first person in Bangkok ever applying for this TR->ED visa change). From application with school to final ED visa stamp took just 14 days, even though schools will tell you it might take up to 5 weeks or longer (they simply have no experience with that yet). Like Tod mentioned, for BKK you need 15 days left on your current TR visa.

The ability to get an in country 90 day ED visa issued is between your school and the immigration office AND is also controlled on how many days you have left on your current entry stamp

For example at Bangkok Immigrations you need 15 days or more remaining on your current entry stamp or extension to apply for an in country Non-Immigrant visa. Whereas at Chiang Mai Immigrations you need 21 days.

You need to be talking to the school you're going to apply at to see IF they can do in country ED visas or not