In my charity work I regularly interact with people whose education has been harmful to them. They failed, and from that failure gained the impression that they were stupid, and/or unable to learn specific subjects. Even putting aside the harm of taking up someone's time to no profit for themselves, you learn more than the information in the curriculum. You learn an attitude towards education, an assessment of how you relate to it. What many people learn is that it's not for them.
http://akaptur.com/blog/2015/10/10/effective-learning-strate...
I think we need more methods for teaching (I wish I had Kahn Academy when growing up!). Everyone gets to where they understand basic math and reading comprehension in their own way, and that's fine. But if we are not checking up on students progress and making sure they have some basic level of general education, we are failing the students.
Harmful? Really? Please tell what harmful education is coming out of standardized tests. Even the standard test taking strategy itself is useful in a general sense. You don't know the solution so narrow down to ones that seem reasonable, pick one and move on.