Tension had been building for weeks--a series of fights between blacks and whites at the city's only public high school, serve, in hindsight, as clear warnings.

But none expected what happened January 7. That Monday, shortly after noon, a black student fatally stabbed a white youth--Anthony Colosimo of East Cambridge--in an unfinished wing of the new high school a block from Harvard Yard.

Within minutes, city officials were on alert, patrolling the city in a grimfaced effort to calm racial tensions before they flared into large-scale fighting.

Monday night, at a community center a few blocks from Colosimo's family home in a mostly white neighborhood, angry residents screamed at school officials. Meanwhile, city authorities manned a crisis center in City Hall all night.

But the city stayed quiet. Gradually, in meeting after meeting, people started to talk. Schools stayed closed for a week, while teachers and students and parents formed committees and drafted resolutions and plans. At the school, officials set up a security system and held workshops, while across the city, clergy and politicians appealed for unity.

When school did reopen, all was peaceful, if not normal. Plain-clothesmen and community leaders walked the halls to quell fights, and the precautions prevented serious confrontations for the rest of the year.

At the same time, around a city once described as a "model of racial harmony," residents asked what had gone wrong.

One answer was suggested the night of the stabbing by teacher's union head Susan Noonan-Forster: "It may be that we wait too long to start mixing kids--it may be that we need to expose them to other races before they get to high school."

Cambridge will act on that suggestion next fall, when school lines are redrawn and pupils bused in an effort to balance racially the city's elementary schools.

So far, none of the anger and resistance that marked Boston's approach to desegregation has marred the Cambridge effort. The School Committee is still drafting final plans, and serious opposition may yet surface, but as one resident said last month, "A lot of people are still remembering what happened to Anthony--they don't want it to happen again."