This article is from the Sacramento FAQ, by David F. Prenatt, Jr. with numerous contributions by others.
From what I have seen of private trade schools, most of them exploit
industrious people who don't have marketable job skills (WITH SOME
VERY NOTABLE EXCEPTIONS). The most successful tradespeople whom I
know have received their training on the job and simply worked their
way up from entry level positions. NOTWITHSTANDING THE HIGH QUALITY
OF INSTRUCTION AT SOME PRIVATE TRADE SCHOOLS, the tuition that most
pupils pay might be better spent elsewhere; junior colleges and adult
schools offer many useful courses that provide technical training
and/or college credit for work experience/internships.