Estancia (13-4-2, 7-1-1 in league) clinched at least a share of the Orange Coast League title and can win it outright by producing a win or tie Wednesday against cross-town rival Costa Mesa.
Defending champion Calvary Chapel (11-5-6, 6-2-1) falls to second place with a game to go.
Estancia clearly wants to finish strong and deliver what Coach Jessica Gatica said would be the school's first outright league title since the year most of the seniors were born — 1993. Estancia shared the league title with Costa Mesa two years ago, in Gatica's first year at the helm.
"I'm just so happy," Gatica said after her team's dominant effort Monday. "The last couple of games we've been possessing well but not attacking well. This time, our possession led to lots of offensive opportunities, which is exactly what we've been working on and exactly what we need to put teams away. Capitalizing on our opportunities has been our weakness, and today we seemed to turn the corner on that.
"They work hard, and it's starting to pay off. It's a good senior day."
Calvary Chapel Coach David Moo said his team was shorthanded early in the game, with two players out due to injury. But he declined to name them or to make excuses, congratulating Gatica with a handshake at midfield after watching Estancia dominate possession.
"Most games, we're laughing and joking around during warm-ups," said Lyons, one of three senior captains along with Freiberger and O'Neil. "This game, no talking and very serious. We wanted to win this one."
Plascencia struck first, in the 10th minute off a corner kick from sophomore Isela Garcia. Plascencia's initial shot from the left side of the box was blocked but she stayed with it, knocking the follow-up into the upper part of the net.
Estancia took a 2-0 lead in the 26th minute, when Freiberger connected from about 25 yards out, off a cross from Lyons. The hard-struck ball slipped out of the fingers of the Calvary Chapel goalie and into the net.