The city’s most educationally and economically vulnerable students could be put into school limbo by last week’s ruling that special-education students who never try public school are ineligible for reimbursement for private-school expenses, a lawyer said.

Historically, the Department of Education has paid for private schooling when it deems students to be so disabled that they couldn’t be appropriately educated in the public schools. The department has also reimbursed thousands of families who have successfully sued the city on the grounds that the public-school program recommended by the city wouldn’t appropriately educate their children.

When Judge George Daniels of Federal District Court in Manhattan found that the family of a boy identified as Gilbert F. didn’t have the right to be reimbursed for tuition because Gilbert had never attended public school, education department officials said the court was helping close a loophole that allowed families to game the system to win free or reduced-cost private education for children who might be fine in public schools.

A private lawyer who was not involved in the case of Gilbert F. but does handle special-education cases, Michelle Kule-Korgood, said that the decision could affect hundreds of families who have been told to send their children to state-approved private schools but cannot find an appropriate space and want to send their children to unapproved private programs instead.

“I think this is going to affect the group of children who are both the most educationally vulnerable and the most economically vulnerable,” she said.

Now, for example, Ms. Kule-Korgood is representing a 17-year-old whose visual and motor skills are average but who on IQ tests scores in the moderately-to-severely-retarded range. In June, he aged out of the state-approved private-school program in which the city placed him.

He then was admitted to an unapproved private school but couldn’t find a space in an approved program. As a result, his lawyer said, since September he has not been attending school.

A spokeswoman for the education department, Michele McManus, said, “As a result of this decision, the department will have the opportunity to provide appropriate services to a student before the parent can seek reimbursement for tuition of a private school.” She said students who attend state-approved private schools at the recommendation of the education department are treated as if they have attended public schools.