Statement of purpose for applying to graduate school in a different field of study than high school
Short answer: It doesn't matter at all.

Particularly in the US, high school degrees don't have "concentrations"; you simply get a high school diploma. In certain circumstances, your school may have a special focus, but accreditation rules for high school diplomas typically specify some distribution of courses which all students have to meet to receive the diploma.

Moreover, the statement of purpose tells us what you want to do in graduate school, why you want to do it, and why you're qualified to do it. That's pretty much it—most people on admissions committees aren't interested in knowing what early pearl of wisdom some random relative imparted, or other "humanizing" anecdote that too many applicants include. In fact, many of them actively hate it. 

Statements of purpose are not your life story, they're a summary of your past and future career. Tell us about your relevant experiences, and why you should be admitted to our graduate program.