An Upper Dublin High School teacher is charged with improper contact with a student.   Investigators say 40-year-old Cheryl Bremble of West Moreland Avenue in Hatboro sent a teenage girl sexually explicit text messages.   They also say she gave the underage girl a sex device on school property.   Bremble was arraigned Friday on charges that include endangering the welfare of a child, corruption of a minor and criminal use of a communication device.   She is free on $25,000 unsecured bail.