The following sentence appears in my textbook.
山下先生はいい先生です。教えるのが上手だし、しんせつだし。
Is the second し at the end of the sentence necessary? Does it change the meaning to omit this second し? Does the second し imply that there could also be other reasons (which are not mentioned) why 山下先生 is a good teacher?
し has two different functions:
彼は背が高いし足も速い。 He is tall and he can run fast.
遅くなったし、もう帰ろう。 It's already late, so let's go back.
だって怖かったし。 Well, coz I was scared.
し often plays these two roles simultaneously. 「お金がないし時間もないし、どこにも行きたくない。」「もう帰ろうよ。暗いし、寒いし。」 The second し in each sentence can be replaced by ので, から, etc. 「お金がないし時間もないので、どこにも行きたくない。」
Anyway, sentence-end し is usually for denoting a reason. In your case, the final し marks a reason why 山下先生 is a good teacher, referring to the previous sentence. Even if you dropped it, the sentence is still understandable, but it's not "optional".
