Teacher: Simmer down, Hermione. Are you talking to me or to that straw man you've constructed? I, and the rest of our present company would have understood that you wanted to borrow a pencil if you had pointed at it and grunted, but I think we can all agree that would hardly be appropriate for this setting.
I know you're in a rush to establish yourself as the smartest person in the room, but perhaps you should have paused to ask yourself if addressing an instructor in an academic setting was the appropriate context to use informal phrasing, particularly when you had reason to believe that this instructor would be inclined to correct you. Usage is dependent on context, and as the author of the original post identified me as "Teacher" for this tiresome bit of Kabuki, it would be irresponsible not to correct you.
If you still need additional reasons, think of it as being comparable to splitting an infinitive or ending a sentence with a preposition. They are not, strictly speaking, prohibited, but they sound like shit and mark you as a buffoon. Amateur sophistry is not going to win you any friends.
No, you can not borrow a pencil, smartass.