Rather than hold its anticipated vote on a plan designed to give high school students more sleep by starting classes later, the School Committee last week instead decided to further study the proposal. As a preliminary step, the committee discarded one of two options it had been considering, voting against a plan that would have middle school students starting at 7:30 a.m., elementary students at 7:50 a.m., and the high school starting classes at 8:32 a.m. During its meeting Monday night, the board agreed to “establish a collaborative process’’ with school district administrators and faculty to examine the remaining proposal, and determine how it might be implemented. Option No. 2, as the committee identified it in its resolution, would have the middle schools start at 7:45 a.m. , the high school at 8:15 a.m., and the elementary schools at 8:35 a.m. Natick High would adjourn at 3:02 p.m. under the timetable. The change would add $118,000 to the school district’s busing expenses, officials have said.