Nicknames could be the same as other people's. Also, most users have more than one nickname, so it is best to use their full name when tagging so you make sure you get the person you want :) 

Although this sounds like a great idea, users usually check for staff members who are around, and if not, then Prefects. To do this people need to look on the map anyway to see who is around to help out if needed, it would take the same amount of time to type @prefects as it would to type @usersname that you have seen on the map. It is also could be abused and annoy the team. 

This differs from site to site and is up to the MoM of each country