It depends. If you're referring to one semester, the apostrophe follows 'semester'. If you're talking about the grades of two or more semesters, the apostrophe follows 'semesters'.
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It depends. If you're referring to one semester, the apostrophe follows 'semester'. If you're talking about the grades of two or more semesters, the apostrophe follows 'semesters'. In each case for the possessive apostrophe, simply find the word that acts as an 'owner'. Put the apostrophe after the 'owner' word, and, if the extra 's' is sounded, include that as well.