So one important fact is that most of the knowledge taught in school is not directly useful. Combine this with the fact that transfer of learning is almost nonexistent (e.g. http://amzn.to/2hExO6u ) and you discover that most education is wasted.

There is extensive evidence of a sheepskin effect, and most studies of educational effectiveness are consistent with signalling theories and ability bias (more able individuals get educated).

https://www.nas.org/articles/The_Sheepskin_Effect

There is likely to be some benefit to education, but it's far less than what politicians currently believe and what current policies are predicated on.