This will be the only S on my transcript
LSAC probably sees more "S" grades than it does "D" or "F" grades, so LSAC needs no explanation. As PPs have said, it won't affect your UGPA.
Now, how potential law schools will view a single "S" on your transcripts is an entirely different question. My guess is that if you're concerned about a "B" hurting your UGPA, you're pretty close to a 4.0. High UGPAs help US News rankings, so I suspect an isolated "S" would be better than something that would lower your UGPA.