Help with rapist?

What a dumb cluck.  You know what she SHOULD HAVE done.  She should still report it to the police - right away.  It is not the job of a school, or the principal to investigate a rape.  That's what the police do.

She wasn’t raped and you don’t have a girlfriend troll

first off, i respect you for trying to help your girlfriend, i hope she's ok. 

depending on where it happened and how aware the guy is there may be forensic evidence left - if he forced her into a room her DNA could be in there, if she bled at some point there may be blood in the house. if the police were to forensically examine the location (idk if they would) and the guy was to outright deny she was there he'd be caught out, more likely he'd say it was consensual (if your girlfriend was below the age of consent at the time he'd still be in trouble for thos).

are there any devices or cameras in the property which might have recorded anything? if he deleted those it'd look suspicious.

others may have made allegations too which would make what she said more believed. Unfortunately trying to get an admission out of him and secretly recording it (check the local laws on this to make sure its admissible in court) may be the only other way and this would be very dangerous.

One thing she might do is talk to her younger sister. Chances are he's had sex with her or tried to. If it turns out he's tried to do it, then there's more ammunition that can be used against him. She does need to report it to the police, though. In the meantime, she needs to get evidence against him such as secret recordings and writing down things that happened and when they happened. If if happens again, she could get pregnant.

Did the guy actually ***, and if so, then did he *** inside your girlfriend? 'Cos if he came inside her, then she might still have some of his D.N.A. inside her. Tell her to go to the police as soon as possible. 

You have to understand first right or wrong he is innocent until proven guilty and he doesn't have to lie either as he can just keep his mouth shut. Remember too he can attack you verbally claiming you are defaming his good character. Those are the bad things if you can do something legally to overcome them you should.

Cool story, bro. 

The very people who could administer a polygraph test would be those police she doesn't want to go see. Polygraphs aren't admissible in court anyway. The evidence window to report a rape is narrow and even when a victim reports right away most D.A.s won't charge a rape case without ironclad DNA evidence. This is why most go unprosecuted. But reporting this is probably the only way to start the paper trail she could use to try to get her sister out of the home. The truth is that nothing's going to happen here unless she gets over her fear of reporting this crime. 

The school has nothing to do with this.  If your girlfriend wants something done, she'll need to report the crime to the police.  

If she doesn't want to go to the police, then NOTHING can be done.