PORTLAND — Under a plan approved by the School Board last week, city schools will add two pre-kindergarten classrooms per year over the next five school years.

In addition, the board backed a proposal by Superintendent Xavier Botana to offer Metro bus vouchers to students who qualify for free or reduced lunch during the first year of the expansion.

The decision will more than double the number of pre-K students. The five-year plan is a first step in the board’s aim of eventually providing universal pre-K.

At least five of the new pre-K classrooms will be in the city’s schools. The remainder, staffed by teachers and educational technicians hired by the School Department, will be at partnering agencies and organizations.

The projected cost to fully implement the pre-K expansion by 2024 is $3.3 million. The district currently spends about $800,000 on pre-K, which was established in 2010.