Well, "doing the best they can" and doing it right aren't the same thing. I can do the best I can to fix your furnace, but since I know jack about HVAC, it's probably not going to get fixed correctly.
Even if you're a great parent, your kids may not follow you lockstep. They are an individual. That's why it's influence and not control. For good or ill, though, it's still tough to argue our parents (if they are around) aren't the biggest influence in our lives for a looong time. It could be positive or negative, and a lot of it is just neutral or what you accept as normal, but they shape the way we see the world and what we learn to question.
I cannot dispute this. However, logically speaking, we cannot assume as you stated that because he was pissed and recorded the teacher it was because of anything you did. It could have been. You (the parents) could have been lousy deadbeats and the kid was emulating what an uncle, neighbor, pastor, or other mentor has taught him.
I realize you're just playing a bit of contrarian here to dampen the outrage. I'm just playing the role of the contrarian's contrarian.