Nicolas Nguyen was found with the gun after school hours in the parking lot, district officials said.
Parents said they were concerned about the news.
"It's very scary and I didn't know anything about it," parent LaTonya Polk said. Usually we get some kind of call or something."
Full statement from Klein ISD:
"On Tuesday evening, a male student, 18 years of age, was arrested by the Klein ISD Police Department and charged with exhibition of a firearm on school property, specifically the campus parking lot. Appropriate disciplinary action will ensue by the Klein ISD administration to ensure the safety of our students and staff."