Ninja sightings cause panic at elementary school, leading to temporary school closure

A ninja sighting at an American elementary school caused panic and even led to the school being temporarily closed.

It's a story I don't really understand, but what does it mean?

Details are as follows.
cbs3.com - Fake Ninja Causes NJ School Lockdown

According to this article, a public elementary school in Barnegat, USA, was temporarily closed after reports of a ninja running through the woods behind the school.

Police confirmed that the 'ninja' was a camp instructor at a nearby junior high school, and that he was wearing a black karate uniform and carrying a plastic sword to participate in a costume event.

Apparently the school was only closed for about 30 minutes, but what kind of image do people overseas have of ninjas?