Formal/Informal Language?

This entry is about a conversation I witnessed between the two adorable boys that clean our office.

Before the actual conversation is written out...some of you may know that the Korean language has both formal (존댓말) and informal expressions (반말). Apparently many of my students don't know that English doesn't really have anything as specific that. Of course, there are phrases that are more polite than others but we don't have formal language that we use for elders and informal that we must use for those younger than us etc...

So back to the boys...they come in everyday to clean my department's office. They usually come in with loud screams of, "HELLLLOOOO!!" and lots of waving. One of the boys comes in late and he yells a big, "HIIIII!!"...to which the other boy scolded him.

He tells him:
Hello! is 존댓말 and Hi! is 반말. How can he use 반말 to a teacher?
To which the other boy says:
What about HI-요? (adding 요 or 니다 to the end of a regular sentence, after conjugating it, usually turns it into formal language)
He responds with:
싸가지 없는것...
Hard to translate this one but I guess it can equate to, "You sound like a jerk/asshole."
They laugh for a bit and then...

The boy turns around to me and screams brightly, "HELLLOOOO!!"

Ah adorable!