LOS ANGELES (KNX 1070 NEWSRADIO)  -- Three Southern California high schools and a middle school were placed on lockdown Friday, but all of the measures were lifted by the afternoon with no serious incidents reported.

One student was arrested at Blair International Baccalaureate School, where a SWAT team was sent shortly before 9:30 a.m. after a student reported seeing someone walking on campus with a gun in his waistband.

Police arrested the male student, whose name was not released, on suspicion of carrying a weapon. He was taken into custody for investigation of being a minor possessing a firearm, police said in a statement.

No weapon was found, but there was probable cause to arrest the student, including witness accounts that the youth was carrying one, Lt. Jari Faukner said.

As a precaution, all 1,200 students were confined to classrooms for hours as police searched the campus, formerly called Blair High School, ``looking room by room, hallway by hallway, bush by bush,'' police Lt. Tom Pederson said.

Anxious parents waited outside for hours until their children began being released after 5 p.m.

Blair East, an adjoining middle school, also was locked down; those students were released at midafternoon. Officials did not search off the campus.

Students were individually searched, then taken in small groups to a gymnasium, walking with hands on their heads. Some parents watching from afar wondered why it was necessary or said they were frustrated by a lack of information.

Other lockdowns occurred during the day at Pete Knight High School in Palmdale, Valencia High School in Placentia, and Kraemer Middle School in Placentia. But no arrests or other incidents were reported on those campuses.