Grade equivalent means that the score is the same as the average score for a person in X grade.

So, for example, if for 10th graders the average score on a particular test is 23 and your kid also gets 23 on the test, then his grade equivalent score will be a 10. Sometimes they also show months. A GE of 10.7 would mean 10th grade, 7th month.

It is extremely common for gifted elementary age students to obtain GEs of 13+.

The scores you've shared are high school grade equivalents. However, they don't mean that your son has mastered high school level material. Most high school students don't master high school level material. For example, a child with mastery of arithmetic and a smidge of prealgebra will have a GE of post high school level on the WJ-III. GE scores >8 really tell you more about what high school students don't know than about what your child does know.

The other issue that GEs obscure is that we don't want our gifted kids to be achieving at an average level when they do high school work. A more telling GE for gifted kids would be the grade level where their performance is at the 90th percentile. After giving my children various tests for a number of years, I believe that the 90th percentile is the level where mastery of grade level material is indicated.

So, for example, my 10 year old son (4th grade age) took the ITBS intended for 7th graders this past spring. I had the scores run four times--as compared to 4th, 5th, 6th, and 7th graders--because I wanted to see what grade level (end of year) his scores were at the 90th percentile (his GEs were all 13+). What I found was that as compared to 4th and 5th graders he was at the 99th percentile, as compared to 6th graders he was at the 98th percentile, and as compared to 7th graders he was at the 91st percentile. So, in my mind, his true grade equivalent is 7.9 (7th grade, end of year). And that happens to match my experience with him (we homeschooled up until this past spring).

Of course, socially an 8th grade placement in a classroom would not work for him, but the scores did make me feel more comfortable with a 6th grade placement with additional acceleration in math.

I hope this makes sense. You son did extremely well!