By Marisa Porges

If current polls prove accurate, the United States will elect its first woman as president this November. She would join 22 female presidents, prime ministers, and chancellors, from Great Britain to South Korea, Germany to Argentina, bringing the total number of female heads of state to 23 out of a possible 195. While a historic moment in the struggle for gender parity, the real story lies in the winding path we took to get here - and how far we have still to go.

Although we tend to celebrate female "firsts," the history of women leading in male-dominated sectors shows that cracking the so-called glass ceiling isn't enough. In most sectors, the glass ceiling was cracked decades ago - only to see few women follow in the footsteps of the pioneers who worked so hard to pave the way. The central lesson for those watching today's elections lies in understanding why it's taken women so long to make lasting headway even after trailblazers provided a model to follow and, in many cases, knocked down barriers to entry.

Few people recall that, in 1932, Hattie Caraway became the first woman elected to the U.S. Senate; it took an additional 60 years until there were more than two women in the 100-person upper house. In 1903, Marie Curie won the Nobel Prize for physics; it took 90-plus years until more than a third of college students graduating with STEM degrees were women. In 1934, Lettie Pate Whitehead became the first woman to serve as director of a major corporation when she was named to Coca-Cola's board of directors; 80 years later, the number of women running Fortune 500 companies hovers around 25, or a mere 5 percent. This despite a growing body of research showing businesses are more successful when they have female executives, since gender diversity correlates with superior corporate performance.

Here's the problem: Once female pioneers break the glass ceiling, there's often a dearth of women waiting in the wings who prove willing, able, and effectively positioned to follow in their footsteps. So while, for the first time, a woman has a real chance of leading the free world, we can't say when or even if we'll approach gender parity in business, science, entrepreneurship, and more.

So what can we do?

Last week, my first as head of an all-girls' school, I saw the answer in the nearly 600 young women who filled the halls. Because every time I saw two classmates helping one another, it was a young woman supporting and mentoring another young woman. Every time there was a problem to solve at the blackboard, or a moment when a student had to direct a club or sporting event, it was a girl taking the lead. Everywhere they look, girls see someone just like themselves taking a seat at the table and speaking from the front of the room. There's no glass ceiling, because they can't imagine what one looks like.

In large part, this reflects the fact that a single-sex environment makes young women more comfortable being themselves, speaking up, and taking charge. Studies have shown that 87 percent of school-aged girls feel their opinions most respected when among their female peers, as compared with 58 percent when in the typical coed school environment. At every point, we are reinforcing the fact that their voices matter - and that they are valued added to any group or situation.

Because even as we celebrate "firsts," we also try to teach every girl that what matters most are "lasts" - when was the last time she spoke up, the last time she supported a friend, or the last time she pushed her limits to try something new. So when they all imagine their futures, they know they should be waiting in the wings for any opportunity that comes their way, whatever path they pursue.

In 1872, Victoria Woodhull became the first woman to run for U.S. president. One hundred forty-four years later, we may finally see the first woman in the Oval Office. As girls return to school this fall, we need to remind them that while female "firsts" matter, it's the "last" that makes the lasting difference.

Marisa Porges is head of school at the Baldwin School in Bryn Mawr.