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I'm not gonna vote because I'm only getting your story. However, if your version is, indeed, the Absolute Reality of an unbiased observer's experience of events ,,, yah. It sucks. Yah, you did right, and yah, you're paying the price for it.
As far as the girl ... it sounds like she's getting it raw. But, you also don't know the entire story from hours or days before. Just saying, keep an open mind that what you observed could well be the culmination of a lot more.
Future reference, if you've got (or friends or anybody nearby) your mobile phone, hit > video > RECORD. You can always help in a situation like that by helping to incite positive reaction in the group around you ... and you don't physically touch. You want to really Do The Right Thing?
Stand between her & her attacker with your hands "in" (near) your pockets and let the other guy beat on you. Don't feed his aggression.
The school admin (rightly or wrongly) has a policy of anyone involved gets the same treatment. The teachers & admin will never know exactly what happened, they only know what others have told them.
The guy gets five days because his mommy threw a temper tantrum ..? Fine. Make sure the ENTIRE school knows that his mommy has to wipe his bottom for him. Make sure word goes out to the entire school that this asshole needs his mommy to finish the fights he loses.
Make a complaint to the School Board (at the District level, not your principal) that punishment was handed down unevenly (don't use the word "unfair"). The administration of your specific school has shown favoritism. Show up at the next two or three public school board meetings and state your opinion verbally and on the public record. Submit a written, detailed complaint requesting investigation.
Make sure to pointedly ask the School Board for all records, including and especially video surveillance, so that your legal adviser(s) may review them.
Just so you know, I was attacked by two thugs on a school bus one day. They were upset that I was sitting in their seat. All three of us were banned from the public bus system for the remainder of the year, even with a dozen witnesses backing up the fact that I was sitting in a seat minding my own business when I was attacked by two older boys.
Life sucks rocks some days. Welcome to ameriKKKa where criminals and thugs get away with fucking up innocent lives.