The loss ended a 13-game winning streak and was Weston’s first since April 11, when it fell to the same opponent.
Weston scored two runs in the top of the sixth inning to take a 3-2 lead but Bunnell scored two in the bottom of the seventh to win it.
Krishna Lal and Tyler Hassett each had two hits and drove in a run for Weston. Ethan Lee Tyson also had two hits.
Starting pitcher Alex McGrath threw six innings, striking out two and walking four in the loss.
Weston plays New Fairfield at home on Monday at 4 p.m. in the South-West Conference quarterfinals.