Moniteau High School was on an exterior lockdown Friday morning as police investigated a suspicious vehicle near the school.
The lockdown lasted for about one hour and only the high school was affected.
Superintendent Sean Arney said classes continued as normal, but no one was allowed in or out of the school. The lockdown was lifted at 11:45 a.m.
State police are investigating.
Arney said students were never evacuated from the high school. An automated phone call went out to parents when the lockdown went into effect, and again once it was lifted.