CARTER COUNTY, Tenn. -

Thursday is the final day of classes at Happy Valley High School, but one final exam put the entire school to test Wednesday morning.

Emergency personnel wanted to gauge the preparedness for responding to crisis situations. "Today has been a learning experience,? said Assistant Director of Schools Kevin Ward.

Just after 9:00 Wednesday morning, a sheriff's department employee entered Happy Valley High School posing as a gunman. The exercise had been planned for months, but the school didn't know specifics.

Sheriff Chris Mathes says while things went well, mistakes were made. "Four of my guys in this scenario would have gotten wounded or killed. That's a bad thing," said Mathes.

If a real event played out like this drill, none of the more than 400 students would have been hurt. They determined communication between responders and the central office needs to be better. "We didn't know what to expect,? said Ward. ?One of the things is communication."

The sheriff says it?s important to test the plans with students present because that's how it would be in real life. ?We know we prevented a Columbine-type situation in our schools a couple of years ago," explained Mathes.

He says two years ago they stopped a plan by a teenager to attack one of the county high schools. The plan included detailed drawings of how many students would be in each classroom at the time. The sheriff says that teen is no longer in the schools, but Wednesday's drill helps make sure plans work in case of an actual attack.

"We learned a lot about the building tactically,? said Mathes.

Emergency crews are planning another surprise drill for sometime next school year at another location. One improvement they want to make after Wednesday's drill is a better way to communicate with students and staff inside the school.

We also learned they want to install a second intercom system that can be used if the main system is disabled.