"We've been through a lot of turmoil, and I don't want to be responsible for continuing the battle and more turmoil in the community," she said in an interview.
Occupying seats on both the council and board could create a conflict of interest and bring on legal challenges, Foley said.
Foley, who has been practicing employment law for 14 years, said that while she's confident the legal battles could be overcome, her time and resources should be spent toward making a difference in Newport-Mesa schools.
Councilwoman Wendy Leece said she'll miss working with Foley, but added that she thought her colleague made the right decision.
"Katrina will be a fantastic asset on the school board," she said. "I was on the school board, and it's more of a family type of a job. Your family is involved in the things you're voting on, and it complemented my family. I think she did the right thing for herself and her family."
Foley's decision to leave hands Costa Mesa off to a conservative-dominated City Council, which will have four Republicans and is likely to appoint a fifth to her seat.
But Foley sees it differently.
"I don't know that I had a liberal view on the council," she said. "My belief is that I represent the residents in a pragmatic, practical way. I've always approached my governing in a pragmatic fashion. It's a non-partisan position; I believe you need to be pragmatic in your approach."
Foley, who defeated incumbent Area 2 Trustee Michael Collier, said she plans to bring a new approach to the school board.
"I'm going to be a proactive board member," she said. "I'm not going to wait for the staff to bring me information, and that's different from what's going on right now. And it's all a matter of how you see your role as a board member. I view it as someone who is responsible for helping to direct priorities and identify priorities."
Collier congratulated Foley, saying she will do a good job.
Councilman-elect Jim Righeimer said he took the position of supporting whatever decision Foley made.
"Her heart is there and that's where she wants to be," he said. "Whatever she wants to do, I'm supportive. She's done a lot of stuff in the community, and she'll continue to do good stuff for this community."
While some might be disappointed with her decision to leave the City Council, Foley said she's not going away and that she'll continue to be part of the community.
"I am sad to leave the City Council because I definitely have developed a really strong bond with a lot of our city staff and our community leaders," she said. "But I still live here and I'm still going to be involved. I am excited about the challenges ahead."