Riley, Northgate principal for 12 years, made the announcement over the intercom at the 1,200-student school and later in a phone message to parents. She said Feb. 29 would be her final day.
"Last year I thought about retiring in June but decided to stay when I found out we had 19 teaching positions to fill," Riley said over the phone. "All of those positions are filled as of today."
An educator for more than 40 years, Riley's retirement comes as parents have petitioned to transfer responsibility for Northgate from Mt. Diablo school district to the Acalanes district.
Riley alerted the district about her plan last week, said Sue Berg, spokeswoman for Mt. Diablo district. While the high turnover at Northgate has been unsettling for the community, Berg said she was sure there will be strong candidates applying to lead Northgate.
An interim principal will be named before Riley leaves, Berg said.