My high school reunion was going amazingly well when the principal called us into a lecture hall.

For the previous hour or so, we had shared hugs and laughs, repeatedly gushing, "You look the saaaame!" as we caught up on the 25 years that had passed since graduating from St. Scholastica, an all-girls Catholic school on Chicago's North Side.

Eerily, almost everything did look the same. Same hospital-clean floors, same mini-gym, same wooden cafeteria chairs.

Even my old perceptions of people seemed in line. The classmate who always made me laugh arrived late and theatrically for the reunion photo, screaming "Wait!" as she ran toward us. The valedictorian-turned-doctor and mother of three was delayed because she had just performed an appendectomy.

We had left our spouses and significant others at home, enhancing the strong sense of unity we women felt. I looked back at our time together in school and wondered how much our deep familiarity, the feelings of trust we shared, were encouraged by not having any boys around. St. Scholastica inspired us to be self-confident at a time when a girl's self-esteem has been known to quietly take leave.

Yet today, enrollment is down at Catholic all-girl schools. And though more public schools are offering an all-girls option, their numbers remain small.

The St. Scholastica principal, after waiting patiently for a bunch of grown women to settle down, delivered a State of the School address loaded with upbeat facts: an impressive college-placement record, an International Baccalaureate program for honor students, a new technology lab.

But the student population had shrunk dramatically, down from roughly 200 girls in my graduating class to about 200 students in the entire school today.

The picture is much the same at girls schools across the city. Twenty-five years ago, there were 29 all-girls schools in the Catholic Archdiocese of Chicago. Now there are 13 such schools.

The shrinking of religious orders, made up of men and women who traditionally taught at these schools for extremely low wages, is an obvious reason for the decline, said Sister Judy Cauley, interim co-superintendent for the Archdiocese of Chicago Catholic Schools.

Another reason is public opinion.

"When we do survey the market, there's an increasing attraction to the coed environment," Cauley said.

One reason Americans are uncomfortable about single-sex schools is that we are afraid to talk about the differences between men and women. We like to think, like my first impressions at the reunion, that everything and everyone is the same.

But things are not always the same or equal in the coed classroom. Nor, for that matter, is my hair really the same color it was 25 years ago.

Sadly, all-girl Catholic schools have declined even as studies have documented that such settings lead to improved test scores, higher-than-average acumen in math and science and increased college enrollment. Self-confidence among girls in single-sex schools also registers higher.

Ever-emerging brain research also makes a case for single-sex schools, pointing out how the learning styles of boys and girls can vary.

I am not an expert on this research, but I can instinctively understand why allowing a young boy to roam a bit freely and make noise in the classroom is not necessarily a bad thing, just as it makes sense to allow girls to work in small groups or arrange chairs in a circle because some of us learn best when we make an emotional connection.

New legislation has made it easier for public schools to offer single-sex education, and the nation has gone from having as few as four single-sex public schools six years ago to roughly 100 today. They include Chicago's Young Women's Leadership Charter School and Urban Prep Charter Academy for Young Men.

Already these schools are showing positive results, particularly for many disadvantaged students who, according to some research, have the most to gain from a single-sex approach.