The Central Bucks School Board and the union representing its bus drivers have agreed that the board can lay off up to 44 drivers at the end of the school year in June. The board wants to phase out the entire transportation staff over the next four or five years to save more than $500,000 a year on payroll and pension costs.  It will contract with First Student, a private company that provides busing for several school districts.  Many of the people laid off will be older drivers who were already thinking about retiring.