Only half the schools the Education Review Office (ERO) visited in the first half of last year succeed in improving the performance of underachievers, it says.
The office said in a report some schools were setting targets that were too general, and they did not have a clear idea of which students needed help.
It reviewed 41 secondary schools and 310 primary schools in the first six months of last year, and found two thirds of them set targets which focused on underachievement.
However, only half of the schools followed their goals up with actions which helped more than 40 percent of the target group make more than a year's progress in their learning.
The schools which were good at helping their underachievers make rapid progress had clear targets and knew which students needed more help, the report said.
"Some of the most successful schools (especially primary schools) set targets for fewer students than the less successful schools," it said.
"They had a clear understanding of who the students were that they needed to target actions to accelerate progress for and were able to monitor their actions to determine if they resulted in positive actions for them.
"In contrast, those that did not do so well had more general goals and some did not follow through on the actions they thought would improve performance."
The report said some schools accelerated the progress of more than 70 percent of their target learners.
The secondary schools it visited last year were generally less effective than the primary schools at using targets and effective interventions.
"They took fewer key actions for success, or did these less effectively, than the successful primary schools," the report said.
"Primary schools have more specific requirements to report each year on the numbers and proportions of students not achieving.
"As a result, boards and leaders of primary schools were generally clearer about the groups of students that needed to accelerate than secondary schools were."
ERO recommended schools clearly identify the students who needed to improve and take a coordinated approach to helping them.