Many parents say they are filled with dread at the prospect of having to go through the public school lottery again after growing comfortable with their schools, which often were not their top choices.

“That’s what keeps me up at night,” said Kristin Barrali, of Roslindale, whose two children could lose access to their current school, Mendell Elementary in Roxbury, under the proposed changes.

“I think kids should be put first in this decision,” Barrali said. “I know money is an issue, but it’s not clear what the cost savings is.”

Initially, school officials said it might be necessary to reshuffle students among dozens of elementary, middle, and K-8 schools across Boston so the city can save money on busing costs. The goal of each of the five student-assignment proposals unveiled last week is to have students attending schools closer to their homes.

But on Tuesday, the officials said it appears the School Committee will have to adopt some kind of “grandfathering” policy to allow some students to remain at their current schools — otherwise students would end up switching schools at an alarming rate.

“We believe the numbers of students who would be impacted without grandfathering would be too disruptive to the city,” said Matthew Wilder, a School Department spokesman, noting that grandfathering current students has emerged as the number one issue raised by parents since the proposals were released.

Under the most aggressive proposal — assigning students to the closest school to their home — more than 17,000 students would lose access to their current school, some 83 percent of all students affected by the changes, according to data the School Department released Tuesday at the Globe’s request.

Even under the most modest proposal, which would carve the city into six assignment zones instead of the current three, more than 7,500 ­students, a third of all students, would no longer live in their school’s attendance zone, ­forcing their parents to apply to a different slate of schools in their new attendance area.

At Wednesday night’s School Committee meeting, Superintendent Carol R. Johnson intends to make some recommendations on grandfathering students at their current schools, Wilder said.

He said the School Department is weighing all possibilities from grandfathering all current students — a commitment that could keep current preschoolers in their K-8 schools for the next decade — to grandfathering for a limited period.

The five proposals represent the biggest potential changes to the way the city assigns students to schools since the current system went into effect in 1989, replacing a court-ordered desegregation plan that polarized the city.

Yet even at that time, the School Committee granted grandfathering to all students enrolled in their school when it voted to adopt the current system.

That promise, however, came with two notable hitches: It guaranteed transportation for only a limited period, and grandfathering did not apply to any siblings who were not already enrolled at the school, according to a Globe story from that time.

Having a high number of students switch schools in a given year could create instability across the system and could hinder academic achievement, said Martin West, an assistant professor at Harvard University’s Graduate School of Education.

“We know the consequences can be the most severe for students who are at the greatest risk of academic failure,” said West, whose research in Florida found that students who transitioned to middle schools tended not to do as well as students who went to K-8 schools.

While he noted there is no proven model to help districts with a large-scale transfer of students, he said Boston could ease the transition by “making sure there is very good communication and transfer of information across schools about students’ ability levels and their needs.”

Academics, though, are not parents’ only concern about losing access to their schools. Many worry the upheaval could shatter a sense of community that many schools have ­painstakingly built over the years, often with the help of a dedicated core of parent volunteers.