Pupils had to hide under their desks after three intruders with knives were seen in a school grounds.
Armed police rushed to North Birmingham Academy as it reportedly went into lockdown.
Officers arrested three teenagers on suspicion of possession of a blade, and there were no reports of injuries.
One parent told Birmingham Live that children as young as 11 were ordered to sit on the floor in their classrooms during the alert.
West Midlands Police said three people had been seen with a knife before some of them ran onto the school grounds on Tuesday morning.
A spokeswoman said: "We were called to reports of three young men seen with a knife in Warren Road, Kingstanding, Birmingham, at just before 9.30am.
The school said it worked with police to resolve the issue, and that the lockdown was "in line with the academy’s procedures".
Some parents said they found out about the lockdown some time after it took place or on social media.
The school said it texted parents to reassure them.
