I would disagree. They maybe don't teach in the way that would be ideal for each individual student (because that isn't possible when classes are in the 10's of students), and some of course are fundamentally terrible as is the same in every single profession, however they provide lots of other value than just teaching. They provide comfort and care and security for those children who have a poor home life. Even if they don't teach the child more than 40 % of what they should teach, what they offer in other ways is more valuable.
