The fights happened on Monday and Tuesday at the same Woodland Hills School District bus stop, WPXI-TV, Pittsburgh, reported.
Police said they're using surveillance video of the fight to help identify those involved; they have so far identified 15 people, including 10 students from Woodland Hills junior and high schools and five adults. Three people -- two teens and one adult -- have been arrested so far.
"Everyone will be arrested. Anyone with any involvement at all will be arrested," Swissvale Police Chief Greg Geppert said.
Geppert said it is not clear what started the fights.
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