Question: My child scored a 100 percent on the TAKS reading test in 3rd grade, what percentile is that? 

Answer: Percentile is supposed to mean the percent of all the people who took the test that you scored HIGHER than.  Since you can't score higher that yourself your percentile shouldn't be 100.  It's usually referred to as 99.9. This assumes that it was impossible to score more than 100% (no bonus questions), and that 100% meant your child got all the questions right.
This question from the Yahoo Q&A forum is categorized in the topic: Education & Reference