This RP is very AU, in that first Sam and Dean aren't even brothers and next its not even set in a Supernatural setting. There will also be underage themes as Sam will be 16 and Dean 18.
Ok, here it goes...
Sam has just transferred to a new private school and things are not going very well for him. Some of the older boys seeing him as 'fresh meat' and begin tormenting him right from the beginning. However the pranks quickly go from just humiliating to violent. Teachers are not any help because they don't believe the boys Sam accuses are doing anything wrong. (Sam is from a poor family and has gotten into the school on a scholarship while the other boys are all from rich families who give a great deal of money to the school.)
Sam doesn't want to tell his father because first his dad is working hard with two jobs to support his family, he can't afford to send Sam to a different private school without the scholarship. And if his dad found out about the abuse, he would take him out and send him to public school instead, and the private school is a huge opportunity for Sam that he won't get again.
After one nasty beating Sam receives from the older boys he is left bleeding and crying in the bathroom and is approached by a younger boy who advises Sam he needs to get a 'sponser' if he's going to survive in this school. Sam doesn't understand what he means and the boy explains the way things work, that sometimes the seniors will give protection to the younger boys in exchange for sexual favors.
Sam doesn't want to resort to that, but he's desperate enough to ask around about it. Unfortunately the boys that are bothering Sam no one seems willing to give him protection from. As a last resort, Sam goes to the one person EVERYONE in the school seems to be afraid of. Dean Winchester.
Of course Sam is terrified because he's seen Dean around the school, has seen how other people react to them, heard people talking about him, and though Dean doesn't get into trouble on school grounds because that would get him expelled, everyone seems to think he's dangerous. But by that point he thinks Dean can't possibly be worse than the group of boys who have been terrorizing him for weeks. When Sam finally catches Dean alone and asks the older boy to 'sponser' him, Dean is first surprised then calls Sam a freak and tells Sam to get the fuck away from him.
Sam is devestated, but resigned, and is not surprised when he goes home from school that day to being surrounded by the older boys who seem to have gotten bored with simply beating him up and want something more now. But to his surprise, Dean pulls up in his car and tells Sam to get in, shocked the boys simply let Sam go. Dean drives Sam home but doesn't say anything to the younger boy's offers of thanks. The next day at school Dean pulls Sam aside and says he'll 'sponser' him. Giving Sam his necklace that he always wears so everyone will know that Sam is 'his' now and not to touch him.
Sam is relieved but also afraid too, because he figures everyone must be scared of Dean for a reason and doesn't know what to expect from Dean now. He's also never been with anyone before, and is not sure what Dean will want from him. But when Dean finally extracts his first 'payment' Sam is shocked by just how gentle Dean is with him and Sam actually enjoys himself. After a little while Sam thinks his problems are over, but really they've just begun.
LOL boy that came out a little more long winded than I planned, but there it is
Enjoy!