BOSTON — BY the end of their first year in a public-school classroom, many teachers have already lowered their expectations, according to a survey of new teachers released this week.The two-part survey polled graduates of teacher-preparation programs before they began their careers and again after their first year in the classroom. Metropolitan Life Insurance Company commissioned Louis Harris and Associates to conduct the survey. "Young teachers have found their job much tougher than they anticipated," says pollster Louis Harris. Before teaching, 83 percent "strongly agreed" that they could "really make a difference" in their students' lives. After their first year, only 68 percent of the teachers "strongly agreed." Overwhelming problems outside the classroom came clearly into focus during the teachers' first year on the job. "The most striking issue in this report is teacher perceptions of their students' readiness to learn," says Keith Geiger, president of the National Education Association. Before teaching, 75 percent agreed that "many children come to school with so many problems that it's very difficult for them to be good students." At the end of the school year, those agreeing increased to 89 percent. In addition, those who said health and social problems should be addressed by agencies outside the school increased from 19 percent before teaching to 25 percent after a year on the job. When the first-year teachers were asked what would have helped them be more effective, 46 percent said an experienced teacher to act as mentor would have helped the most. "New teachers are not getting the help they need ... [they] want feedback from someone who knows the ropes," says Albert Shanker, president of the American Federation of Teachers. Both before and after their teaching experience, nearly all the teachers agreed that working with parents is essential. But 71 percent said parents treat the school and its teachers as adversaries. Despite all this, an unchanged 89 percent of teachers agreed that if they do their job well, "their students will benefit regardless of how the rest of the school functions."