(Gresham, OR)  --  Gresham police say they've arrested a student at Sam Barlow High School after the youth was accused of being on the school's roof with guns and ammunition.  Authorities say the arrest took place Saturday, after multiple students reported seeing a suspicious person on the roof of the school during its annual winter formal dance.  Police say the student told officers he was on the roof in a "security capacity" to ensure the safety of students at Sam Barlow's  "Snowball Dance."  KOIN-TV reports the student is facing multiple weapons-possession charges.