As others have said theres no way all those 50 will fight, when I first went to secondary there were lots of problems with the yr 9's trying to control the school. So our year which was made up of a lot of 'hard' people started fights with them. We needed to get about 25 people together to guarantee 2 would fight, the other side about the same

. During school people like to act hard, so they'll gather together the strongest guys will dive in, most likely no one will follow, and if the strong guys get knocked back 100% chance no one else will follow. When the fight happens ignore the crowd aroun, the guy that steps forward is your only target, drop him and assuming your friends have done likewise with any others who step forward it will be all over. To concentrate on the crowd would be a mistake, allowing there strongest guys to get in and hit you down.
School age fights could be fought of as a large game of dodgeball with slightly modified rules, every time you hit a memeber of the opposite team one more of your team will be allowed into the field of play, and 3 of the opposing team will have to step out. Think of the crowd as out of play, a handful of people on each team will start, throw your punches at the players in play, you hit one, several the others who came forward will step out, one more on your side will step in. But obviously you allow the other teams in players to be succesful then more of theres will run in, and your 'friends' will abandon you. This is true in 99% of school age fights your teams 'strong guys' are winning then people on your side will rush in to kick the weakened teams guys; your strong guys losing, everyone on your side backs off.
Don't think 50 people vs 15, think 3 vs 3; you can win it easily. People do not want to fight unless they think they can win, this continues throughout life but is empasisied in the school mentality. Good on your mum for saying just lay them out, its the best way forward; I know I'm closer to school age then several other members and not as mature but I remember well the difference between standing to in a fight, even if you lose, and calling in the parents/teachers in terms of permanently ending the problem or just postponing it.