While doing your first visit to the school you will come across a various groups of gingers. Loot every ginger that you find and eventually you will find an item called Ginger Freckles (a certain ginger drops them I just don't remember which one, but if you loot them all it is impossible to miss as it's always dropped). Put these on then proceed to get the gold key and unlock the door to detention. Once you do that you will be forced into a battle with the Boss Hall Monitor.