If you're a full time student, you're not unemployed, even if you have a semester or a summer off. As long as you're in a program of full time study, for background check purposes, you don't need to explain anything.
Typically you're only expected to explain gaps in employment, not counting time as a student or as a minor (under 18) that occurred in the past 5 years, and lasted two consecutive months or longer. If you have had any such periods, it's not an automatic out, since there are many valid reasons to be unemployed. For example, you could be caring for a sick family member, or you could be on a self-discovery voyage around the world (some employers actually respect this reason). The trick is, whatever explanation you give should sound like you were alright before, but your time off was either for the greater good (caring for someone) or added to your personal value (upsell your personal brand in this case). Be that as it may, if you had just a few months off, once, over a year ago, then "the job market was bad, I had to move to find work, and decided it was a good time to get back into school" is perfectly acceptable.
Don't sweat it.