A special committee exploring changes to the way Boston assigns students to public schools has modified one of the proposals school officials presented last month. The panel decided at a meeting Monday night to split in half a student assignment zone that encompassed much of Dorchester and Mattapan, which would have offered more than a dozen choices but would have made it less predictable for parents to know which school their children would eventually attend. The change means that a proposal for a 10-zone student assignment system will now call for 11 zones. The two other proposals under consideration, developed by researchers at MIT and other institutions, remain unchanged. That’s because those proposals have no student assignment zones and instead rely on an algorithm to generate a list of school choices for each family based on the distance from their home. The panel plans to endorse one of the proposals by the end of this month. A community meeting will be held on the proposals Wednesday at 6 p.m. at the Trotter Elementary School.