While a lot of what is being said here is generally correct, some is not universally so.
1) Look across all schools, enrollments and sports, and larger schools don't ALWAYS win.
2) While "demographics" are relevant, the far more telling predictor APPEARS to be access to superior club coaching WELL BEFORE HIGH SCHOOL AGE.
3) Some parents WILL move to put a soccer-playing child in a certain school, or at least consider such factors when doing a relo.
4) Sport "supremacy" runs in cycles. During my 30 or so years in Charleston, I've seen great boys' soccer programs at Summerville, the old Middleton, West Ashley and Wando. And not so long ago, I seem to recall West Ashley winning a girls' state championship.
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Regarding a new high school in Mount Pleasant, CCSD appears committed to finishing its various rebuilds of older and seismically-endangered schools BEFORE adding a new high school. From what I have been told, there is a possibility "Old Wando" will be remodeled as a ninth-grade annex, leaving only one HS in Mount P. The other possibility is that a totally new school -- with a Class AA-AAA enrollment would be built or opened.