Rat Boys are basically roadmen that are in your school. These boys are the ones who shout random words in the halls while bounce-walking. If you go to secondary school in the UK, these are the boys that are in 5th and 6th year, "Weasel Boys" are in 3rd and 4th year and "Mouse Boys" are the ones in 1st and 2nd year. The only thing you need to look out for when trying to find a Rat Boy, just look for the roadmen in the school.
Example
Rat Boy 1: "EL CHAPO!!!"
Rat Boy 2: "OHHHHH, YOU WANT TO GO OUT FOR A CIGGY?"
Rat Boy 1: "YEAH BRUV, BUT FIRST WE GOTTA GET SOME UNDERAGE SKETS!"
Rat Boy 2: "FRICK YES MATE! LETS GOOOOOOOOO!!!!"