Do you want to go to school to learn thai or for a visa? There's a difference. If you don't truly want to learn thai then don't go to school and get another visa. If you do truly want to learn thai then go to a school based in an area where the overwhelming majority of the locals speak only thai.
Is your girlfriend a long term government employee who owns her own house and has all her taxes up to date, or, is she a freelancer, massage or bar girl that does everything in cash?