Three more people have been arrested after two related incidents in Mandurah and Coodanup on Tuesday.
At around 6.40pm, a fight broke out at the Mandurah Aquatic and Recreation Centre.
Three hours later, police were called to another disturbance in Coodanup involving a group related to the earlier incident.
A 17-year-old boy from Mandurah has been charged with aggravated assault occasioning bodily harm and taking part in a riot.
He will appear before the Juvenile Justice Team.
A 12-year-old girl from Mandurah has been charged with participating in a riot and was issued a Juvenile Caution.
A 15-year-old girl from Mandurah has been charged with participating in a riot and threatening behaviour and was also issued a Juvenile Caution.
Police inquiries are continuing.