A coalition including the city teachers union, the principals union, and parent activists is lining up to oppose Mayor Bloomberg’s proposal to raise the standards for entering high school.

The proposal, which Mr. Bloomberg announced last month in his State of the City address, would prevent eighth-graders from moving to ninth grade if they scored poorly on state tests or fail to pass certain core classes. Modeled on a policy now in place for third-, fifth-, and seventh-graders, students who do not meet the criteria would be forced to attend summer school.

The proposal faces an approval vote at the next meeting of the city’s Panel on Educational Policy. The parent group, the Coalition for Educational Justice, recently released a report recommending three changes for middle schools: an extra 90 minutes in the school day; mentoring programs for first-year teachers, and professional counselors to build in emotional and social support.

The group is charging that the cost of the mayor’s proposal, an estimated $118 million, would be better used to lower class sizes, extend the school day by 90 minutes, and introduce school in August for sixth-graders.

A Department of Education spokesman, David Cantor, said: “Of course the schools have to be supported. But kids also shouldn’t be leaving those schools who have no chance to succeed in high school.”