A couple of questions. Asking specifically about Chiang Mai.

When can the extension be processed?

1 week before? 1 month before? 1 week after? What are the acceptable periods to extend?

Does the extension need to be done in person? can the school or visa agent process the extension?

I am considering changing schools after 1 year so I don't have to commute on the dangerous roads.

Is it possible to change schools without going to Vientiane Laos etc to get a new visa. Can I change schools without leaving Thailand?

You can do an extension up to 30 days early. Late will result in an overstay.

You can change schools without needing to get a new visa. You need to have paperwork from new school and a letter from old school stating that you are no longer enrolled in their school effective the date you apply for a new extension.