Corona del Mar High seniors Kacie Kline and Kendall Mulvaney could feel the clock ticking early Tuesday afternoon.

Kline and Mulvaney were in their Advanced Placement exam for art history, which started at noon and was scheduled to last until 3:30 p.m.

That time frame is long enough, without knowing that you have a U.S. Lacrosse Southern Section South Division playoff game that starts at 3:15. Kline and Mulvaney could have taken a rescheduled exam, but CdM art history teacher John Gunnin advised them not to do so, as the rescheduled exam is more difficult.

So Kline made a quick judgment call Tuesday after finishing her exam.

"I left," she said. "I just walked out, and I'm pretty sure my score got canceled. Kendall, they didn't let her leave, but I just got up and walked out ... In [the playoffs] everything is so much more intensified. That could have been our last game."

The two team captains indeed made sure they extended their high school careers at least one more game. Kline had four goals and three assists as No. 6-seeded CdM topped No. 11-seeded Mission Viejo, 16-10.

CdM (15-4), which shared the Pacific Coast League title with Beckman, plays at No. 3-seeded Foothill in the South Division (Orange County) quarterfinals on Thursday. Foothill defeated Temecula Valley, 15-6, in another first-round game Tuesday.

The Sea Kings will be looking for an upset similar to when they topped Foothill, 16-15, in a nonleague game April 14. At the time, Foothill was ranked No. 1 in the state by laxpower.com.

The first-round match was tenuous early. Kline got to the field less than a minute into the contest, entering the game soon afterward following a quick warmup. When Mulvaney entered the game with just more than 10 minutes left in the first half, CdM had a slim 4-3 lead.

"When they didn't let me leave [the test] I went to the bathroom like six times, put on my cleats, did some jogs and some stretching [in the hallway]," Mulvaney said. "I knew Kacie was warming up and the game had already started. We were just trying to make the best of it, and we did."

Mission Viejo (9-8) tied the score at 4-4 when junior Sam Leyds scored off a free-position shot with 8:45 left in the first half. But the Sea Kings showed that two could play that game.

Freshman Kennedy Mulvaney, Kline, senior co-captain Sabrina Smith and sophomore Payton Carter scored consecutive free-position goals of their own before the first half ended. CdM opened up an 8-4 lead.

Carter continued her strong play overall, tallying three goals, five ground balls and five draw controls. Senior co-captain Jamie Smith, Sabrina Smith, Kennedy Mulvaney and Paige Miller all scored twice for the Sea Kings, with Kennedy Mulvaney leading everyone with six ground balls. Her twin sister, Katherine, also had a couple of key ground balls in front of the CdM net in the second half.

Senior goalie Kate Allen made five of her six saves after halftime for the Sea Kings, who never were seriously threatened in the second half. They celebrated after the game ended, singing "Happy Birthday" to assistant coach Jessica Murray.

Kendall Mulvaney had a goal and two assists for the winners, who scored seven of their goals on the free-position shot.

"I think our speed is an advantage," CdM Coach Aly Simons said. "More than anything, I think our transitional passing up the field was really good this game. The ball moves so much faster than if you're running, so it's almost unstoppable."

Marissa Meissner led the Diablos with four goals, and Leyds had three. Freshman goalie Elizabeth Scott had a strong game with 12 saves.

CdM has had success against Foothill. Two years ago, the No. 7-seeded Sea Kings upset No. 2-seeded Foothill in the quarterfinals on their way to the South Division championship game. Last year, CdM upset Foothill in a nonleague game.

Simons said she hopes Thursday's game can be played with three referees. The Sea Kings hired a third referee for Tuesday's contest in preparation.

"Calls are made, and the game is safer with three refs," Simons said. "We're an aggressive team and Foothill's an aggressive team, so we want to keep it as safe as possible."

Kline called Thursday's game "giant."

"[The earlier nonleague game] was a very aggressive, emotional game for both teams," Kline said. "I think we're just trying to mentally prepare for that, and just stay composed. Aly and Jess always say our biggest advantage is us staying composed, and letting the other team break apart."

matthew.szabo@latimes.com

Twitter: @mjszabo