For several months, the subprime-mortgage crisis has hovered over colleges, its ultimate effects uncertain. That is changing. In the past few weeks, a series of loan companies have withdrawn from broad categories of student lending, and some colleges have acknowledged sharp increases in borrowing costs. The most severe effect may be still to come, as an unknown number of students--faced with higher costs, tougher loan conditions, and their own mortgage troubles--could decide that they simply can not afford college anymore. According to John D. Walda, president of the National Association of College and University Business Officers, the problems facing student-loan companies, and universities struggling to pay for major capital expenditures, are proving to be very big in terms of impact on families and on institutions. Yet the potential loss of students because of family financial crises "would really be No. 1" on his list of effects of the mortgage crisis.