North Korea's military power is...?

My teacher gave us the impression that N. Korea's army was like a lot of cavemen with M16's trying to throw grenades across the ocean at the US. However, I didn't believe it so I've been looking it up and I've read their army is in the top 10?! I'm so confused, why are they so high up and still have not been able to succesfully nuke us (thank the Lord), excluding the fact there is an ocean dividing us. I thought, according to my teacher, they weren't very progressive. I need some enlightenment, thanks!