SACO — This was one upset where the disbelief extended to both sides.

Sanford High, a field hockey team that starts five sophomores and two freshmen, stunned sixth-ranked Thornton Academy 3-2 in the Western Class A preliminary round Saturday at Hill Stadium, getting the winning goal from sophomore Makayla Garvin in the fourth set of penalty corners.

“I’m, like, dumbfounded,’’ said Sanford Coach Diana Walker. “I can’t believe it. I mean, some of these kids didn’t even know what the tournament was.

“I have a very young group, just three seniors. And those seniors have progressively worked with them all year long, and they’ve gained more confidence and worked together and just did it.’’

Sanford (8-7), seeded 11th, will face third-ranked Falmouth in the quarterfinals either Tuesday or Wednesday. Thornton, which beat Sanford 1-0 in the regular season, finished 11-4.

The Trojans controlled much of regulation, getting a rebound goal from Libby Pomerleau in the first half for a 1-0 lead. But the Trojans lost defensive leader Francesca Petrucci to a back injury and the Spartans began to attack. They tied it with 3:16 remaining on a goal by sophomore Elizabeth Helmreich.

“When we tied it up, I thought this was a great game,’’ said Walker. “Anything that these young kids would get from this game would be great.’’

The Trojans had a great chance to win at the end of the second overtime, but Sanford goalie Sarah Goldthwaite, also a sophomore, stopped Lauren Russell point-blank.

Then on to penalty corners, where the game officials had to consult the Maine Principals’ Association handbook to check the rules.

Sanford appeared to win it on the third set of penalty corners when Helmreich scored on the Spartans’ second penalty corner and Thornton was held scoreless after its two corners. But the officials allowed a third opportunity for the Trojans and Kaitlin Hilton put in a shot at the post to tie it again.

No matter, the Spartans struck on the fourth set of corners again, this one on their second set. On the goal, Garvin got the ball on the left and fired it through a screen into the goal.

“I don’t even know right now, it just happened,’’ she said. “I just had it. There were two people there and I just lifted it to get over both of their sticks and it went between their goalie’s legs.’’

The Spartans’ defense, led by senior Emily Salls, stopped the Trojans’ final chance to earn the quarterfinal berth.

“It’s not about being tired,’’ said Garvin. “It’s emotional, you don’t know what to think.’’

Trojans Coach Lori Smith said her team didn’t play as well defensively as it had all season, especially after Petrucci went out. But she gave the Spartans credit for being aggressive.

“We had our chances,’’ she said, “but that’s field hockey.’’

Mike Lowe can be reached at 791-6422 or at:mlowe@pressherald.comTwitter: MikeLowePPH