Serena Rivera-Korver scored four goals, Georgia Lewis recorded 11 saves and Castilleja beat visiting Menlo-Atherton, 12-6, to clinch the top seed in the upcoming PAL tournament.
The Gators (5-0, 13-4) still have a league match remaining at Menlo (4 p.m.) next Wednesday, but Woodside, which beat visiting Burlingame, 14-8, is the only team that can match Castilleja and the Wildcats (4-1, 6-6) lost, at home, to the Gators last month.
Claire Pisani added three goals and Anika Adzich had two for Castilleja.
On the boys side, Menlo-Atherton handed visiting Half Moon Bay its first PAL loss, 11-1, on Wednesday and Woodside earned its first league victory, beating visiting Mills, 14-8, as Giorgio Fatica and Levi Vigdorchik each scored five goals.
The Cougars (3-1, 7-10) plays at Hillsdale on Monday before finishing the regular season at undefeated Menlo School on Wednesday. The Bears (4-1, 10-8) host Mills next week.
Castilleja received solid rounds from its three sophomores Wednesday and the Gators beat Sacred Heart Prep, 213-225, in West Bay Athletic League action.
Divya Tadimeti earned medalist honors with a 2-over 38 at the Stanford Golf Course. She was followed by Niav Layton at 40 and Alyssa Sales at 41.
SH Prep senior Caroline Park scored a 40 in challenging course conditions.
Castilleja (8-2) plays Menlo at the Menlo Country Club on Monday at 3:10 p.m. while Sacred Heart Prep (4-5, 5-6) hosts Notre Dame San Jose at Shoreline.
Julia Owens won at No. 2 singles and the duo of Lauren Corfield and Elena He won at No. 2 doubles but host Pinewood fell to visiting Valley Christian, 5-2, in a nonleague match Wednesday.
The Panthers (4-11) return to WBAL play Thursday with a match at Menlo at 3:30 p.m.