>>21
And drilling is a lot easier if you have more motivation than you would if you were doing it alone, such as...I don't know...A test? Yeah.
I did learn kanji on my own, but it's still hard, and a lot harder for people who aren't very disciplined.
There is also some degree of knowledge on how to teach that is required, obviously.
Tests help, though, especially when they're actually being graded. It does vary by person, but if there is an option between a class and self-study, the class is almost always the better choice.