The use of the tassel (and its status) on the school caps appears to have changed over time. So far the story appears to look like this

Pre 1951. (Inputs from Michael Boon)
Sub-prefect - braid on either side of front panel
Prefect - braid along all panels
Colours - tassel for rugby, alternate white panels for cricket and nothing for athletics Read More...

Tags: Caps, Prefects, Sub-prefects, Colours, Chricket, Rugby, Athletics