Student prepares to sue school district for high-speed 'promposal' that led to crash: VIDEO

CHANDLER, Ariz. -- What was supposed to be a harmless traffic prank left a motorcyclist injured, and it was all caught on camera.

On Tuesday morning, heart-stopping video shows a so called "promposal" on a school campus gone terribly wrong.

The city and school district in Chandler, Arizona are now facing the prospect of a million-dollar lawsuit.

The video shows a Porsche driven, police said, by a student with his girlfriend in the passenger seat, barreling through a stop sign into a motorcycle. The rider, 18-year-old William Vannasap, was thrown into the air, plummeting to the ground.

"It's just heartbreaking even to watch that video, because in any instant, he could have passed away," said Mayci Vannasap, William's sister. "He told us that when he got hit, he let out the most blood-grueling scream ever."

The crash was so intense that the airbags of the car deployed.

"You can see on the video the student wasn't going 15 mph in the school zone. You could tell that he was going way faster," said Daisee Vannasap, William's other sister. "That was very shocking to us."

William has filed a notice of claim against the school district and the city, stating the student driving the Porsche had permission from the school resource officer and the principal to run a stop sign in the school parking lot as part of the student's plan to ask a girl to an upcoming school dance.

The notice of claim said the officer would pretend the driver was in trouble for running the stop sign. The driver would then pull out a poster, asking his girlfriend to prom.

"Well, its private property and minimal injury," a school resource officer is heard saying in the video.

"I know, but like, I was supposed to go through the stop sign. That was the thing," the Porsche driver said.

"Yeah? We'll talk about that later, but it's private property so I won't be giving anyone tickets or anything like that," the officer said.

The principal was seen in that school resource officer's patrol car moments after the crash.

The officer stated in a police report that the principal had approached the car to discuss a parent issue involving a separate incident, and the officer told him he was helping a student in a prom proposal. In the report, the officer stated the plan was for the driver to roll the stop sign, but instead, the driver drove towards the stop sign at a high rate of speed and did not look both ways nor slow down.

The district told ABC News it is unable to comment, as this is a pending legal matter.