There are some major criticisms of language schools (such as this one by Tim Ferriss) and, to be honest, a lot of these criticisms are valid.
I’ve been to a few language schools in Bangkok with the intention of learning Thai, and the quality of teaching varies wildly. Several teachers at various schools scolded me for taking the initiative to teach myself to read and write Thai while living in London. I was surprised at how negative their responses were!
But eventually I did find a school here in Bangkok that was right for me.