Gov. Charlie Baker is unveiling legislation he says will let the state add up to a dozen new or expanded charter schools each year outside an existing cap on the schools.

Baker says the bill will focus the new growth in the lowest performing districts.

The bill unveiled Thursday would also let the schools use a system that gives additional weight to high-need and low-income students, as well as students learning English and students with learning disabilities.

Baker, a long-time charter schools supporter, says the legislation will help give every child the opportunity to a high-quality education regardless of their zip code or background.

About 80 charter schools currently operate in Massachusetts.

Opponents say the proposal, which echoes the language of a proposed ballot question, would undermine public schools.