About half of the student body at an elementary school in Santa Clara stayed home today due to a bomb threat that caused a heightened police presence at the school, Santa Clara Unified School District spokeswoman Tabitha Kappeler-Hurley said.On Aug. 4, officials at Briarwood Elementary found graffiti on a door on campus that threatened a bomb would be set off at the school today, Kappeler-Hurley said. Santa Clara police have been on campus throughout Thursday night and today. A bomb-sniffing police dog also went through the entire campus and did not detect any explosive materials, she said. School started at 8:40 a.m. and Kappeler-Hurley said that by noon, roughly 200 students were absent. About 420 students attend Briarwood. No arrests have been made in connection with the bomb threat, according to Kappeler-Hurley. Monday was the first day of the school year at the school, located at 1930 Townsend Ave., she said.