"We were at a stop sign in Georgetown, Mississippi, on Highway 27 and this bus was on Highway 28 with smoke coming from underneath it," Windom, 24, a postal worker, tells PEOPLE.
Shocked, the siblings from Wesson, Mississippi, waited a few seconds to see if the bus pulled over. When it didn't, they took off after it instead of continuing home, beeping and waving and flashing their lights to get the bus to stop.
"I was hanging out the window trying to get their attention," Insley, 29, tells PEOPLE.
When pulling up beside them didn't do any good either, Windom tried something a bit more drastic.
"I just pulled in front of them and slammed on the brakes to get them to stop," she says. "That's when I saw sparks underneath the bus so I knew something was going to happen."
Windom got out of the car and started yelling, "Get off the bus!"
Inside, librarian Chelshe Archie, who was chaperoning the 21 Port Gibson High School seniors on their way back from state exams, sprang into action.
"My first reaction, honestly, as a teacher was to make sure all the kids were off the bus," Archie, 34, tells PEOPLE. "I was extremely worried about my kids."
Not long after she got the last kid off, with some of them begging to go back to get their purses and backpacks, the bus burst into flames.
"All of a sudden it just went boom!" says Archie.
Archie, the kids and their families are grateful to the sisters for saving them from what could have been a terrible tragedy.
"They are real heroes," says Margaret Goods, whose 18-year-old granddaughter Keonna Goods was on board, "because in my mind just the thought of thinking your child could have burned on the bus is more than you could really bear."
Heroism and HumilityWindom and Insley insist they're not heroes.
"Our parents raised us the right way," says Insley, a nurse. "We saw someone who needed help and we helped them. We're happy everyone's okay."
School officials believe a mechanical problem caused the bus to burst into flames, WFAB in Baton Rouge, Louisiana, reports.
On Wednesday, the sisters will each receive resolutions thanking them at the school board meeting, says Jayne Sargent, conservator of the Claiborne County School District in Port Gibson.
"The way they stepped in was just wonderful," she says. "I can't thank them enough."