I'm sure you would find some duds in schools catered to a single goal. Some would be good.
Schools that work with mechanical skills would be the ones that I am assuming would be worth it (ie: welding school or something) since the skills are pretty much just industry standard basics that everyone has to know. IMO a place like this would be more for the practice of the skill and making sure you learn how to do it in a safe manner / the way its expected of you.
I would disregard any sort of trade school rooted in a liberal arts, like stand up comedy, or something that is almost entirely memorization, like bartending.