BOSTON — Boston school officials and union leaders have reached an agreement to add 40 minutes to the elementary and middle school day, Mayor Marty Walsh announced Friday.

The agreement would affect 60 schools, or nearly 23,000 kindergarten through eighth-grade students. It also would double the amount of time teachers have for planning and development.

The longer school days would cost about $12.5 million a year once fully implemented, and teachers would earn an additional $4,464 a year, according to the Walsh administration. The teachers union membership and the city school committee will have to approve the agreement.

The mayor said in a statement that students have a better chance at success the more learning time they have.

Boston Teacher’s Union President Richard Stutman said the extended school day will benefit art, music, drama, foreign language and other under-served subjects.

Currently, elementary students typically have six hours of class time and middle school students have 6 hours and 10 minutes. High schoolers, who are not included in the proposal, have classes for 6 hours and 30 minutes a day.

If approved, the district would roll out the longer school days over three years, beginning with about 20 schools in the 2015-2016 academic year.

The plan adds up to about an extra month of learning per year, Walsh said.

The National Center on Time & Learning, a Boston-based nonprofit organization that advocates for longer school days, applauded the proposal.

“With this agreement, Boston is emerging as one of the country’s leaders as it embarks on a plan to strategically redesign and expand learning time for both teachers and students,” center officials said in a statement.