User blog comment:Supereinstien001/Was James Potter Head Boy?/@comment-4643582-20120331170307

Being a prefect is not a pre-requisite for being Head Boy or Head Girl. To use an American example, you do not have to be a state governor to be elected President, although it helps as a number of Presidents were perviously governors.

The desired skill set for the two positions also is different. The best choice for mayor (Head Boy/Girl) of a small town is not necessariy the best choice to be police chief (prefect).