Rose Grant taught 4,000 students in her decades of teaching science at Bishop Stang High School in Dartmouth, but Sean Gannon and his family always stuck out.“To remember these kids — I know their names, I kept in touch — it says something about them that they made that kind of impression,” Grant said.Gannon, a 32-year-old Yarmouth police officer, was killed on duty Thursday in Marstons Mills.When he was in high school, Grant said, Gannon was a hardworking, dedicated student.