Bell Word Puns

So I have a class where I sit in the band hall and practice my instrument, and near the end of class I looked at the clock and noticed that the bell should have rang already (at 10:40). I checked my phone, and it was only 10:37.

The conversation went like this:

"Mr. Band director, that clock is ahead."

"No, that clock is a clock." (I didn't get it)

"What? No it's ahead."

"No, your head is a head. That is a clock."

I persisted (because I STILL didn't get it) and he said to the other people in the room "can we all agree that that is a clock, and not a head?"

I finally got it, and said "It's a clock, but it's also ahead - like one word - ahead."

"No, the clock is running fast, but it is not a head."

My friend - "it can't run because it doesn't have any legs."