The six were wounded Tuesday shortly after a group of Mojave High School students got off a school bus in a working-class neighborhood in northeastern Las Vegas, the AP reports.
The investigation was focused at Mojave, but security also was increased elsewhere, said Clark County School District police Lt. Ken Young.
"We've got extra officers at several high schools," Young said.
Police were search for at least two suspects. The attackers were believed to have fled the scene of the shooting on foot.
Police said they believed the shooting was linked to a fight at Mojave earlier in the day Tuesday. The brawl had been contained by school police, who arrested three male students, all about 16, officials said.
Sheriff Doug Gillespie did not describe how he believed the incidents were connected. The fight at the school did not appear to be gang or race-related, he said.
"At this point, the indicators that we have found ... it's a dispute over a girlfriend," Gillespie told reporters.
Alejandro Rios, 18, who was shot in the torso, was upgraded from critical to serious condition early Wednesday, University Medical Center spokeswoman Cheryl Persinger said. A 17-year-old male also hospitalized had been listed in serious condition late Tuesday, but an update was not available because he has chosen to keep his information private, Persinger said.
A 21-year-old man and three Mojave High students suffered minor injuries and were treated and released in the hours after the incident. Police said the four who suffered minor injuries had just gotten off the bus, but they would not say whether the two victims still hospitalized were students or had been on the bus.
Police believe the attackers had been waiting at the bus stop. Gillespie said the shooting did not appear to be random.