A teenager was arrested after striking a teacher with a snowball at a high school where he had been banned, Bridgeport officials said. School officials and the teacher told WTNH-TV that the student trespassed into Central High School on Tuesday and threw a rock-hard snowball in a classroom that hit her on the left side of her head. She said she felt dazed and sought treatment at a walk-in medical clinic. Officials did not name the student or say why he was banned from the school. They said he transferred to another city high school. The teacher also was not identified.