Originally Posted by Mr Spooner

This tradition comes from back when planes had 1-2 seating, meaning instead of sitting side by side with the instructor, the student would sit in the single front seat and the instructor would sit behind them in the single back seat. Whenever the instructor would need to talk to the student in flight he would have to get their attention to put the ear piece next to their ear to listen, they would do this by pulling on the back of their shirt tail to get their attention. So now that they soloed they do not need a shirt tail anymore for the instructor so it is cut off.