Students filed through metal detectors and bomb-sniffing dogs roamed the campus after another threat at Academic Magnet High School.
Fifty-five students did not show up for classes Tuesday. Their parents met with police and school officials in the afternoon.
The school has received several threats since Thursday. The most recent came on Monday, officials said.
North Charleston police have turned their attention to parents, asking them to call in any information they hear, even if it is simply a rumor. Parents are in a unique position to ask their children questions and to monitor their conversations and online activities, police said.
A student apparently tried to communicate with authorities Tuesday morning by writing "There is NO bomb threat" on a wall in the boy's bathroom, according to an e-mail sent to parents Tuesday.
A host of security measures are in place until the culprit(s) are caught. Officials are randomly checking bookbags, security officers are stationed at all outside doors and students must log in and out of their classrooms.
"We're going to take it day by day and keep these security measures in place," said Elliot Smalley, executive director of communications for the Charleston County School District.