i did see that part, and more. she arrived same time but left with the friend before police could identify the underage drunk person (her friend). The girl did do the right thing by not letting her freind drive drunk, but that's just a smoke screen for the real reason and why the school is punishing her.
Again you are playing the assumption game. For all you know the police were more then aware of the drunk girls state but let her go with her sober friend that they did recognize. Iv personally been on both sides of that fence when I was under age and was allowed to both go home with a sober designated driver as well as be one taking drunk underage friends home. Shockingly police can be sensible and not resort to arresting everyone they see doing something wrong in the eyes of the law.