I don't think the OP suggested that the role has anything to do with academic ability. As often as not the prefect role is a way of teachers recognising different skills in children. The academically able get rewards for that, the sporty ones win on sports days and in competitions or represent the school on teams. The ones who can act or who are confident get role in plays, the arty ones have work on display etc.
IMO a caring school should look for ways to reward every child for something, so they all have a stake in the school community one way or another. A prefect can fulfil that role with or without abilities in other, more obviously recognisable, talents.
Congratulations, OP. You must be very proud of your granddaughter, and she should be proud of herself, too.