1) The sound you make when you enter the school building.
2) The sound you make during group works when someone tries to make an actual contribution but you just can't let them finish. Mostly followed by apologising and asking them to repeat what they said, just to find yourself interrupting them by making the sound again.
3) The sound you make after school when you feel completely done with everything.
1) *Enters the school building* 'Ahhhhhhhh!'
2) 'Well, I thought we could start with the author and then move on to the-' - 'Ahhhhhhhh!...I'm sorry! What were you trying to say?' - 'I said, that i think that we should start with the-' - 'Ahhhhhhhh!'
The word to use when you don't know what to use. This word can be used in many cases such as a scarymoment, when you forget or don't know what to say, or even (my personal favorite) when the room is extremely silent and you want to make some friends.
Example:
CLASS: (completely silent reading their books)
YOU: Ahhhhhh!
CLASS: I respect you! You have broken the silence and ruined our concentration! I want to be your best friend!