No, not quite. In your example, scoring at the 87th percentile with 30/30 correct means about 13% of first graders scored 30/30 on the reading. Percentile ranks indicate what proportion of the norming group had scores that were lower than the test taker. So a percentile rank of 90 means that 90 percent of the norming group scored the same or lower on the test, percentile rank of 50 means that half of the norming group did the same or worse. A score at the 95th percentile is in the top 5% of performance with respect to the norming group.
