A trophy boy is a boy a girl will hook up or sleep with because although she may not have feelings for him, he is extremely hot and it will look good on her reputation.
If a grade 9 girl were to get with a grade 12 guy, he would be her trophy boy.
Not that there is such a thing as just casual sex for a girl...unless she's a slut because then it's for money.
Gossip Queen: Dude, did you hear? That grade 9 girl fucked a grade 12 boy!
Other Girl: Wow, that must be some trophy boy for her!