What should i expect during a tribunal...?

okay so i got into a fight at school. right now im doing 10 days oss and a tribunal this wednesday. she hit me first after i attempted to walk away. & she has messages on her phone of me confronting her & cussing at her. im also recieving a charge letter today. whats that? also wat consequences do you think ill face at this tribunal? wat do they usually do at a tribual?