School administrators were alerted to the threat this morning after it was posted online by a student at another school.
The threatening Snapchat posts were not deemed credible by school officials, but for more than two hours, no one was allowed in or out of school.
Dozens of frightened parents rushed to the campus to make sure their students were okay, five days after the deadly school shooting in Parkland, Florida.
"I work 30 minutes away from here, and I was just driving crazy, like shaking, 'cause what happened over there in Miami, I believe," parent Marcella Torres said.
Shaouri Garza, an eighth grader at Stevenson, described the tense situation inside her classroom as the lockdown continued into the morning.
WATCH: Threat 'unfounded' after lockdown at Stevenson
"We had to hide under the desk, we couldn't get out," Garza said. "It was kind of scary, I was just wondering who were the people that were trying to shoot down [the school]."
HISD officials said the student who allegedly made the threat was found sick at home with his mother. The student did not have any weapons.
Authorities said the student has been disciplined, but did not give specifics in the child's punishment. His name is not being released because he is a minor.
Parents were notified after no threat was found, but many parents are still wondering what took school officials so long.
"People are in a panic mood. You just want to see your kid safe," said parent Dietre Huff, who picked up her child from Stevenson. "You want to send them off to school safe, you want them to be safe and when things like this happen, of course you're going to panic."