The Sheriff's Office says 27-year-old Andrew Gerhardt was arrested Tuesday and charged with disseminating obscene material to a person under 18. He has been released on his own recognizance.
Deputies say Gerhardt's face and genitalia can be seen in the photo sent July 14.
According to an incident report, the girl's parents found the photo after taking her phone away as a punishment for breaking curfew.
Dorchester 2 spokeswoman Pat Raynor says the district gave Gerhardt a notice of recommended dismissal.
Gerhardt teaches physical education and coaches junior varsity women's basketball at Ashley Ridge High School.
A phone number listed for Gerhardt was not in service.