When Jerry D. Weast became the superintendent of the Montgomery County, Maryland, public schools in 1999, he spent the summer poring over student-achievement results and demographic trends. Then he created a map to illustrate what he'd found. The map divided the suburban district, just outside the nation's capital, into two distinct areas, which he dubbed the "red zone" and the "green zone." Most poor, minority, and English-language learners lived in the red zone, an urbanized core that was attracting a growing immigrant population. The green zone was predominantly white, affluent, and English-speaking. Academic performance closely mirrored the demographic trends, with the lowest-performing schools overwhelmingly concentrated in the red zone. Without swift and deliberate action, the district faced the prospect of becoming split in two, divided by opportunity. To Mr. Weast, the solution was obvious. Montgomery County needed a differentiated strategy that funneled extra attention and resources to schools in the red zone, while increasing academic rigor for everyone. Since then, Montgomery County's leaders have maintained a delicate balance between "raising the bar and closing the gap" that has enabled the nation's 16th-largest school district to narrow achievement gaps while retaining the support of wealthier, highly educated parents. But as the district moves onto the much tougher shoals of middle school reform, that balance could be sorely tested.