LANSING (AP) — Third-graders who started school in recent weeks may be forced to repeat the grade next year if they struggle with reading, the result of a 2016 Michigan law whose most controversial provision kicks in beginning next spring.

The number of third-graders who are held back is expected to rise.

Nearly 700, or 0.67%, of the state’s 100,000-plus third-graders were retained in the last academic year. Researchers at Michigan State University estimate that between 2,000 and 5,000 students, or 2% to 5%, may be required to repeat third grade under the terms of the law.

Between 7% and 11% of African-American third-graders may be held back. The retention policy may affect 12% to 20% of third-graders attending certain low-performing schools.

How many kids are held back will depend on whether those who score more than a grade level behind on reading skills as part of the M-STEP’s English language arts test are given a “good-cause” exemption.

“Ultimately it’s at the discretion of the superintendent. How the superintendents apply this law locally is going to be a huge deal,” said Josh Cowen, co-director of MSU’s Education Policy Innovation Collaborative, which will join the University of Michigan and the state to evaluate the law’s impact on students and make recommendations.

The exemption will apply to students with disabilities, those for whom English is a second language, students who got intensive reading and were held back previously, and those who have been at the school for less than two years and did not receive an individualized reading intervention in their old district. Superintendents or their designees also can still promote children to fourth grade if they determine it is in their “best interests.”

“This isn’t a superintendent sitting in his office just making a random decision. This is based on input and based on what’s best for the child,” said Chris Wigent, executive director of the Michigan Association of Superintendents & Administrators.

The law’s most contentious piece — retention — is new. But other components have been in place for a few years, including increased state spending on literacy coaches, K-3 reading tests and instructional time for children who need extra help reading.

Wigent said in most cases teachers have been able to identify kids who struggle with reading in earlier grades and give them extra help. But he noted that not every child stays in the same district or school from kindergarten through third grade.

It may make sense to let students proceed to fourth grade even if they struggle with reading, he said. The law requires those who get an exemption to still participate in a reading intervention program.