discuss, learn, plan, practice, use and reflect (Aschbacher, 1993).

Just as time can influence the content selected for instruction and how it is taught, it also can influence the type, frequency and thoroughness of assessments. Carving out the time to engage in ongoing assessment of all students requires a shift in the conceptual understanding of the teaching process itself. Darling-Hammond and colleagues (1995) conducted case studies of five schools where teachers and students throughout the school were actively engaged in formative assessment. In describing the shifts in instruction that were supportive of improved classroom assessment, Darling-Hammond and her colleagues maintain that teachers moved from the role of instructors to that of facilitators. As facilitators, the teachers created learning opportunities for their students that encouraged students to engage in their own work. This provided teachers with the opportunity to observe students' work, to talk with them about what they were learning, and to use these observations to inform their teaching.

Regular time also needs to be provided during the school day for teachers to take part in professional growth activities, conduct research on their teaching practices, observe other classrooms, use available external resources, and attend professional meetings and conferences (NRC, 1996).

Although several states have recently implemented strategies to reduce the number of students assigned to classrooms at the primary and elementary levels, most middle and high school classrooms are still over-crowded. Additionally, with departmentalized courses in many middle and secondary schools, teachers often find themselves teaching 150 or more students per day. When faced with large numbers of students and other site obligations, teachers may have a difficult time maintaining continuous classroom assessments.

However, because assessment information is such a powerful tool for supporting student understanding and learning, even science teachers with large classes can find ways to incorporate multiple ongoing assessment strategies into their instructional activities. Student self-assessment, for example, is not only an essential tool for developing student self-directed learning, it also can provide a means for teachers with large classes to successfully incorporate ongoing assessment practices into their instruction. Through self-assessment, students are able to reflect on, internalize, and take responsibility for their own learning. With the teacher serving as consultant, students develop scoring rubrics and criteria to judge their own and their peers ' work.