Update 11:25 a.m. Dec. 20: Police say the contraband was two bullets on a bus and no other ammunition was discovered. (statement added to original story)

Original story:

Administrators at Summerville High's official Facebook page have posted a message to allay fears as students tweeted about a lockdown due to a gun on campus — some saying students have left the school after the incident. 

The Facebook page reported that "contraband" was found on a school bus, and a thorough investigation revealed everything was secure. Summerville Police have identified the contraband as bullets, releasing this statement:

Today 12/20/2012, the Summerville Police Department was notified a bus driver had found two bullets on a bus. In an abundance of caution, Summerville High School officials removed the students from classes and they conducted a safety check of the students and their lockers. No further ammunition or weapons were located. 

A statement from the district will be available soon.

Here is the statement from the Facebook page:

Parents, we extended first period this morning due to contraband found on one of our buses. Summerville Police, district security, and the SHS admin team conducted a thorough investigation and found nothing to be alarmed about. We have now returned to our normal exam schedule. We hope you have a great holiday break!