For a great many young men and women, this is the time of year for a fateful decision_not quite up there with choosing a partner for life, perhaps, but close. That is the decision about where to go to college, and as one who went through the parental agonies of this last year, my heartfelt commiseration to those going through it now.

A recent survey in USA Today points up a particularly hazardous dimension of this whole business for young women. The study, done by University of Illinois researchers Terry Denny and Karen Arnold and presented at the American Educational Research Association meeting in Chicago, found a sharp decline in self-esteem and estimates of their own intelligence among top female students after they had been exposed to a year of college.

The study of 45 women and 36 men, who were valedictorians, salutatorians and honor students in high school, found that 23 percent of the men and 21 percent of the women saw themselves as "far above average in intelligence" when they were high school seniors, but by the time they became college sophomores, only 4 percent of the women still rated themselves at that level, while 22 percent of the men did. Among both men and women in the group, 7 percent rated themselves as "average" intelligence while in high school. By their sophomore year in college, 9 percent of the men still thought they were average but 27 percent of the women in that group did.

Arnold, echoing a theme sounded by other researchers who have studied the treatment of women in higher education, told USA Today: "They're not getting the same recognition, the chance to use their skills, the potential employers seeking after them, the chance to try things."

By contrast, consider the results of a recent study by the Women's College Coalition of nearly 5,000 graduates from the classes of 1967 and 1977 at 48 women's colleges conducted last spring. The study was intended to examine the undergraduate and postgraduate lives of two groups of women and their attitudes toward their college experience and subsequent activities. The typical woman in the study group was a liberal arts major who did some postgraduate work, is employed full time and married. Her salary is in the $20,000 to $30,000 range, although one-fifth of them earned more than $35,000.

The coalition study found that 71 percent of the alumnae would attend the same college again and two-thirds of them would go to a women's college again. They found their colleges to be highly responsive to the changes brought about by the women's movement and gave them high ratings on such issues as fostering self-confidence, bringing successful women from outside into the institution, and encouraging students toward careers. The women were nearly unanimous, according to the coalition, in agreeing that women's colleges promote self-confidence among their students.

Nearly 90 percent of the students were involved in at least one undergraduate activity, and more than 40 percent held student leadership positions. More than three-quarters of them continued involvement with their schools through some form of alumnae activity.

The case for women's colleges is fairly simple: they are places where women come first. They do not have to compete with men in the classroom for an instructor's time or attention; they can run for president of the student council, not secretary or treasurer, and they will be the editors of the campus newspaper, not news editors. They have ample opportunities to learn leadership and to express themselves and engage in intellectual experimentation without fear of criticism or ridicule from men in the classroom. They do not have to share athletic resources with men.

The majority of women's colleges are headed by women, and women hold a higher proportion of tenured faculty and top administrative positions there than they do at coeducational institutions. Women students are constantly exposed to role models of female achievement. Women's colleges have been in the vanguard of creating women's studies courses, and both formal and informal programs designed to prepare students for the complex job of balancing family and career, which the majority of them face.

Probably at no other time in this century, at least, have American women had both the career opportunities and the challenges in their personal lives that they face today. For many, college is their last chance to train for that challenge. The last thing they need is a college experience that leaves them with diminished expectations and a diminished sense of their own value.