School officials are tweaking one of three models that a mayoral advisory group is weighing as it seeks to overhaul the city’s school assignment system. A 10-zone model will become an 11- zone model, with Dorchester getting split in two, one of the advisory group’s members, Bill Walczak, said Tuesday night.

The other two models are called “home-based” and do not include zones, instead deploying an algorithm. Walczak said the decision to add another zone to the 10-zone model was driven by Dorchester’s original zone being too big and students having a minimum of 17 schools available.

Splitting the Dorchester zone in half allows for more localized predictability about which schools a student might potentially attend, he said. Under the 11-zone model, the chance of students attending the same school as their peers from their neighborhood increases 37 percent, compared to 24 percent under the 10-zone plan. All three models – the 11-zone and the 2 “home-based” models – remain on the table for the 27-member advisory group, known as the External Advisory Committee, as it considers changes to the current 3-zone system. Walczak said several members of the advisory group remain intrigued by the “home-based” models because of their flexibility.

Mayor Thomas Menino has asked the panel to finish up its work before the end of the month and submit its recommendation to Superintendent Carol Johnson, who will take it to the mayorally appointed School Committee.

The advisory group has planned community meetings, including one that was scheduled for last night at Trotter Elementary School and another in East Boston next Thursday, Feb. 21. The panel will have a regular meeting next Tuesday at Suffolk University at 6 p.m.