Can I change my ED visa to a Non-B visa without leaving Thailand if I get a teaching job?

If on the initial visa then no it's not possible. If on an extension of stay it is possible but apparently quite difficult and approval is not guaranteed. I was advised its much easier to exit Thailand to apply and then re enter in the non b visa

No it's not possible to change from an ED visa to a Non-B visa inside Thailand. If your actually on an extension of stay based on attending school it *might* be possible to change the reason for your extension but only with the cooperation of you employer, most don't want to supply the extensive paperwork required to do this inside Thailand and require you to apply for the visa outside Thailand where the paperwork required is much less