Michelle Obama is the former first lady of the United States. The opinions expressed in this commentary are hers. View more opinion articles on CNN.

(CNN) On my final trip overseas as first lady, I visited a high school in Unification Town, a small village about an hour outside Liberia's capital city of Monrovia. There, I sat in a classroom where the only light came from the cloudy, rainy sky outside. As heavier clouds rolled in, the entire room dimmed, and I began to find it difficult to make out faces across even a short distance.

Michelle Obama

What I remember most from that day, though, couldn't be more clear: It's the promise inside each of those girls -- girls who show up every day to learn. They show up even after walking for miles or waking up early to earn some extra money to help pay their school fees. They show up even though their families depend on them to take care of younger siblings, cook meals, and ensure their household is running smoothly. They show up even though many are pressured to marry as adolescents, sidetracking their own goals for a man's.

The girls in that school are joined by millions of others who aren't able to get an education at all -- today, more than 98 million adolescent girls around the world are not in school. The reasons for this are many, including scarce resources, early pregnancies, dangerous commutes, and threats of violence.

Equally pernicious is something they're taught from an early age -- the belief that because they're girls, they're simply unworthy of an education. It's the same toxic mindset that keeps girls here in the United States from believing they can become computer scientists or CEOs. And it's a mindset that together, we've got to change.

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That's why today, on the International Day of the Girl, I'm proud to announce the Obama Foundation's launch of the Global Girls Alliance . We're seeking to empower adolescent girls around the world through education, so that they can support their families, communities and countries. The evidence is clear. Girls who attend secondary school earn higher salaries, have lower infant and maternal mortality rates, and are less likely to contract malaria and HIV. And studies have shown that educating girls isn't just good for the girls, it's good for all of us.

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