<Ul> <Li> A quarter system divides the academic year into four terms, one per season, with attendance required in three quarters per year to total 32 to 36 weeks of instruction . </Li> <Li> A semester system divides the academic year into two terms of equal length, with attendance required in both semesters to total 32 to 36 weeks of instruction . There is often an optional summer session half as long as a full semester . </Li> <Li> A trimester system divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction . The trimester evolved from the semester system in the 1960s and, thus, is compatible with it (i.e. it is simply a semester shortened from 16--18 weeks to 14--16 weeks). The spring - summer trimester may be a full trimester or may be divided into distinct spring and summer half - trimester or shorter sessions, in which classes meet double - time (or greater) to provide the same instruction that would be received in a full trimester . </Li> </Ul> <Li> A quarter system divides the academic year into four terms, one per season, with attendance required in three quarters per year to total 32 to 36 weeks of instruction . </Li> <Li> A semester system divides the academic year into two terms of equal length, with attendance required in both semesters to total 32 to 36 weeks of instruction . There is often an optional summer session half as long as a full semester . </Li> <Li> A trimester system divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction . The trimester evolved from the semester system in the 1960s and, thus, is compatible with it (i.e. it is simply a semester shortened from 16--18 weeks to 14--16 weeks). The spring - summer trimester may be a full trimester or may be divided into distinct spring and summer half - trimester or shorter sessions, in which classes meet double - time (or greater) to provide the same instruction that would be received in a full trimester . </Li>

How many months are in a high school semester
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