LAKE WYLIE, S.C.—This lakefront suburb of Charlotte, N.C., is among the Sunbelt’s strongest magnets for young families.

Since 2000, Lake Wylie has tripled in population to 12,000 on the strength of its good schools, low taxes and proximity to Charlotte’s jobs in the financial and technology sectors. But those schools are filling up, the water system frequently fails under increased demand and 20-mile commutes are stretching to 90 minutes.

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