After three years of getting only A’s or B’s on its city-issued report cards, PS 377 in Bushwick sank like a stone this year — hitting rock bottom among city schools with an F.

It’s ranking tanked from the top-seventh percentile of elementary schools in 2009 to dead last in 2011, spurred largely by faltering test scores.

Students dropped 12 percentage points since last year on the state’s math tests and 6 percentage points in reading — leaving less than 1 in 3 kids on grade level in both subjects.

Seventh-year principal Dominic Zagami did not respond to an e-mail seeking comment.