Grade equivalents on norm-referenced tests are not generally reliable for determining instructional level. They tell you what score the average child of that grade-level would obtain on this specific set of tasks, but do not translate directly to the instructional level of the test-taker.
1. The median child of a certain grade-level may perform above or below the expected instructional level for that grade-level.
2. Even though the number of items correct is the same, the actual correct items that resulted in this score may differ between the test-taker and the median child of the nominal grade-equivalent.
3. Nationally norm-referenced tests are only a sampling of the skills expected in any local educational community; as they are not comprehensive, there is no measure of mastery/non-mastery for any of the (majority of) skills not assessed.
Percentiles will tell you only the rank-order comparison of your child's performance in each skill area to that of children in the norm group. They will have the same deficits as far as sampling, and a lack of correspondence between median children and instructional expectations.
If you want a better measure of instructional level, I would suggest a criterion-referenced measure, such as end-of-course or placement assessments from the curriculum actually in use in your school system.
