Boston Public School officials are planning to roll out, over the next three to four years, a new curriculum for first- and second-graders as well as more summer school options and after-school partnerships with local organizations as part of an effort to improve early learning, close the achievement gap and support working families.The new program will be offered to all of the district’s 250 first-grade classrooms next year while a pilot program is rolled out for some second-grade classes.