Classes were canceled Monday at the central campus after a chemical device was detonated at the freshman school.
Sixteen students and one staff member were slightly injured in the small blast Monday.
Authorities are calling it a serious, credible and specific threat.
As students left school, bomb-sniffing dogs arrived. For the second day in a row, Lockport police are dealing with threatening incidents at the local high school.
"The call was again computer generated, specifically a threat to east campus at a specific time today," said Garry Raymond, superintendent, Lockport Township High school.
As a precaution, students at the east and central campuses were sent home, a day after a chemical-filled bottle exploded inside the freshman campus, sending several students to the hospital.
"Currently they're all separate. But you know, one incident may lead to the other. We just don't have that connection yet," said Chief Bill Kendziora, Lockport Police.
Last week a bomb threat was written on the wall inside the east campus but it was determined to be a hoax.
All three incidents have parents concerned and students mad.
"I'm feeling confused and scared to send my son to school. It's definitely not safe and this is a small town and a close town and this is kind of a shock too," said Jennifer Siefert, parent.
"It's not fair to us because we have to make it up," said Cecilia Huante, student.
Though it is not known if Monday and Tuesday's acts were related, officials believe both were committed by students and they're relying on students to give them the information they need to solve the crime.
"We really need to get to the bottom of this quickly and put it to rest," said Raymond.
The east and central campuses will reopen Wednesday morning but with a heightened level of security. Students will not be age to bring backpacks, water bottles or small containers into the classrooms. They'll have to leave all that inside their lockers.
The Crime Stoppers of Will County are offering up a $5,000 reward in both incidents.
Police are still trying to determine exactly what kind of a chemical device was used Monday and exactly who did it.
Police were on campus all day at Lockport Township's central campus Monday to do a thorough search. They had not found anymore devices.
The incident began about 7:40 a.m. Monday morning.
The school week had barely begun when police and the fire department were called to the freshman campus for what was first reported as an explosion in the second floor hallway.
"We didn't hear anything, we didn't smell anything, we didn't see anything. We didn't even know anything was going on until they literally had to tell us," said Kayla Tran, student.
It was a plastic bottle with some type of chemical inside.
"The initial reports were that a black smoke appeared and some fluid that they got on their skins," said Chief Bill Kendizora, Lockport Police Department.
Sixteen students and one teacher were treated and eventually released for their injuries.
The school was placed on lockdown for more than two hours while investigators made sure everyone was safe.
"I knew if it was something serious, like a gas leak or something we would have been out of the building so I wasn't really concerned," said Thomas Hatzopoulas, student.
While students remained calm inside, parents panicked outside, trying to get information and also trying through cell phones to speak with their kids.
"Did we hear from the school? No, OK, we had to hear it from the media that our children were in danger. And I'm sorry, I got a problem with that," said Kathleen Zidek, parent.
Around 11:30 a.m. on Monday morning, the lockdown ended and students were dismissed for the day.
Students believe another student set off the chemical device.
"Any time we have injuries to students and staff, I wouldn't consider that a prank at all. So we are doing everything we can to find out who is the perpetrator of this incident," said Kendizora.