Yes, you are right, school do not train for slavery literally. But that wasn't the point. As you've recognized yourself, it was an analogy.
As an analogy, you can't say it is invalid by stating differences between the analogues, all analogies are pretty much aware of these differences. If not, they would be identities, not analogies.
"Juvenile", "disgusting" and "ludicrous" are not arguments sufficient to invalidate the analogy either.
What you have to do to invalidate the analogy is to refute the similarities between schooling and slavery, some of them:
- kids are forced to go to school;
- kids are brainwashed to think that school is the only alternative they have besides starving and ultimately dying after an horrible life;
- kids are trained to be submissive to teachers and other in liderance positions.
As an analogy, you can't say it is invalid by stating differences between the analogues, all analogies are pretty much aware of these differences. If not, they would be identities, not analogies.
"Juvenile", "disgusting" and "ludicrous" are not arguments sufficient to invalidate the analogy either.
What you have to do to invalidate the analogy is to refute the similarities between schooling and slavery, some of them: