TRACY-- Tracy's two high schools enjoyed uninterrupted classes Tuesday, the first full day of school since five bomb threats prompted four evacuations in less than a week.

Two Tracy High School students, not named because they are minors, were arrested Thursday in connection to threats made March 13 at their school, and police have identified two other teen suspects in threats to Kimball High, police Chief Gary Hampton said.

Police first thought the threats were a hoax to disrupt last week's state-required high school exit exams but now believe the thrill of the hoax may be playing an equal role.

"Candidly, we think that these are all hoaxes being played out by students," Hampton said. "The thing that is probably the most painful for me to accept is that our kids are playing on our emotions from the loss of a bunch of kindergartners and first-graders (at Sandy Hook Elementary) who were murdered in a classroom, and they're capitalizing on the massive state of alert so they don't have to take a test.

"In light of Sandy Hook ... no police department can react in any way other than to take every one of these threats seriously," he said.

Ominous graffiti on the wall of a girls bathroom at Kimball stirred initial rumors of threats against the school on March 8, and students took to Twitter to discuss threats of a possible shooting, said Tracy schools spokeswoman Jessica Cardoza. By March 11, rumors talked of a hit list.

Substantiated threats began last Wednesday at Tracy High, where bomb threats appeared on the wall of two boys bathrooms, one suggesting "Tracy High is next," Cardoza said. On Thursday, a verbal bomb threat against Kimball High was called in at Tracy High.

All students and staff were evacuated in both incidents, Cardoza said, and police and bomb-sniffing dogs inspected the campuses.

No threats or evacuations were reported Friday, though Tracy High students spent most of the first period in the gym while dogs checked the building.

Kimball High remained quiet until early Monday, when another threat was left in the boys locker room.

Similar threats were phoned in Monday to a Fremont middle school.

Hampton did not rule out the possibility that threats to Tracy, Pleasanton and Fremont schools were connected.

"A lot of this has been unfolding on social media," Hampton said. "Who knows if this was a planned event?"

Parents were required to show identification and pick up their students after each Tracy evacuation. Many were frustrated by long lines at congested check-out points, with waits ranging from a half-hour at Tracy High to almost two hours at Kimball.

"During the school day, students are the responsibility of the district," Cardoza said. "We are charged with protecting them and keeping them safe, and we must verify that all students are checked out to a parent or guardian."

She said checkouts at Kimball were particular logistical problems because there is only one road in and out, and the area is under construction.

"It's not necessarily the process itself -- there are great traffic obstacles to getting thousands of parents out at one time," Cardoza said.

Beyond check-out frustrations, Hampton warns about further, more serious problems for families of teens involved.

"We are having to consume valuable public safety resources to address these hoaxes," he said. "While suspects could be criminally culpable, there is civil liability for the parents."

The threats prevented even the most prepared students from taking the exams Hampton calls "springboards for higher education," but it is students taking part in the hoax who risk their future, he said.

Contact Erin Ivie at eivie@bayareanewsgroup.com. Follow her at Twitter.com/erin_ivie.