what reason did the boys give Scout for she could not go swimming with them

The boys are skinny dipping and Scout has lost her place as "one of the boys." She is in fact a girl, and they've finally reached a point where she 'can't' be one of them all the time :-D

Jem tells scoutt there going in naked so she cant come with them. this shows that Jem and Scout are growing a part because Jem is growing older