When referring to levels of education, what does High school 13 mean?

Question

or high school 14 or 15.

Answer 1

I'm not sure but I've heard the first year of college being referred to as Grade 13 because it's basically like high school.  

Answer 2

what the heck are you talking about??  

Answer 3

We would call it the 13th grade when someone went to the local community college.