Not all trade schools work to reduce the supply of labor. If you hold a trade school that teaches people to be a plumber or carpenter and you don't artificially

Not all trade schools work to reduce the supply of labor. If you hold a trade school that teaches people to be a plumber or carpenter and you don't artificially constrain who can and can't work as a carpenter or plumber then this isn't behaving like a guild. Certifications are more complicated - definitely some instances of certifications function as little more than to reduce the supply of labor. But there are some cases where wanting to certify some level of competence may be desirable.