On Monday the girl was struck across the head with a firework in the High Street.
Then on Tuesday lit fireworks were thrown at police officers and members of the public on the same street.
One officer was hit in the face with a firework but his injuries are not serious.
Officers are investigating whether the disorder in the High Street on Tuesday is connected to similar nuisance the day beforehand.
Police detained the 16-year-old from the Kingstanding area yesterday on suspicion of assaulting the girl. He has been bailed while enquiries continue.
Additional officers have been put on town centre patrols and are working with partners to prevent similar disorder.
Police said they have seen no repeat since Tuesday.
Anyone with information is urged to contact police via Live Chat at west-midlands.police.uk between 8am and midnight, or call 101 anytime.
Crimestoppers can be called anonymously on 0800 555 111. Quote 20SW/256450T/20.