Students who attend Portland’s three high schools won’t be riding in traditional yellow school buses this fall if, as expected, a proposal that has the full support of the School Board receives formal approval this spring.

Board members on Tuesday night authorized the formation of a citizen task force and gave Superintendent Emmanuel Caulk the go-ahead to develop a cost-sharing agreement with Portland Metro that would allow the bus service, which serves riders in Portland, Falmouth and Westbrook, to transport high school students to and from classes.

The new transportation system will give the state’s largest school district more flexibility to adjust class start and end times for its students, with the biggest change coming at Portland High School, Deering High School, and Casco Bay High School. Instead of starting their day at 8 a.m., the board is proposing that high schoolers start their day at 8:35 a.m.

The change in high school start times is part of a larger plan to extend the school day by 20 minutes for all students.

Both changes must be approved by the School Board at a regular meeting. Chairwoman Sarah Thompson took an informal poll of her colleagues – asking literally for a thumbs up or thumbs down – at Tuesday night’s workshop.

While there was some disagreement about the high school start times – some board members supported an 8:55 a.m. start for high schoolers – there was total agreement on letting Metro take over high school student transportation needs.

“This is a game changer for us,” Caulk told the board. “A partnership with Metro will greatly increase our (bus) capacity.”

Talks between the schools and Metro started about a year ago and under a tentative proposal high school students would be given a Metro bus pass.

Greg Jordan, Metro’s general manager, said Metro will offer students a reduced boarding fare of 75 cents. The current boarding fare is $1.50. Under the cost-sharing agreement, Portland schools would cover the cost of the boarding fares, estimated to be about $160,000 in the first year of operation. The bus pass could be used only during the school year.

Jordan estimates that about 600 students, or 30 percent of the high school student population, will sign up for a bus pass. Under current school policy, high school students living within 2 miles of any high school are ineligible for school transportation. The new system would allow a student within such an area to ride to school on a Metro bus.

Jordan said Metro views the program as a way to build ridership. He said it will also give parents greater transportation options for their children and will reduce traffic congestion.

A task force made up of students, parents and teachers would have to review the cost-sharing agreement before the School Board votes to ratify it. Thompson said the board will also hold public forums on the proposal before any final action is taken.

“I think it is a wonderful idea,” said Laurie Davis, a School Board member. “And you are right. It will build ridership.”

Ed Suslovic, a Portland city councilor who also serves as president of Portland Metro’s board of directors, said Metro will offer a phone app that students can use to track the location of their Metro bus.

“Each student will be able to determine in real time where their bus is,” Suslovic said. “When we presented this to students, they got very excited.”

Suslovic added: “We really see this as a partnership and we are building the ridership of the future.”

Caulk said the school department will still be responsible for transporting high school students to and from field trips and sporting events that occur outside Metro’s service area.

“This is absolutely fabulous,” said School Board member Marnie Morrione. “A year ago this seemed like a pie in the sky idea. It’s the type of thing that big cities do all the time.”

The School Board had difficulty Tuesday night agreeing on new class start and end times, but after a lengthy discussion informally decided to have high school students start their day at 8:35 a.m. and end at 3:05 p.m. Under the current class schedule, high school students begin their day at 8 a.m. and end it at 2:10 p.m.

Middle school students will see a slight change in their start times, from 8:25 a.m. to 8:15 a.m. Only King Middle School will be different, with a start time of 7:55 a.m., according to Caulk.

Elementary school students will start their day at 8:35 a.m. or 8:55 a.m. depending on which school a student attends.

The changes in start and end times are being driven by a provision in the new Portland Education Association contract, which contains language that seeks to increase student instructional time by 20 minutes per day. Experts say that additional class time gives students the chance for more help, allows for a reinforced math and science curriculum, and provides the opportunity for more electives such as art and music.