"Her older sister is a high school student."
You guys are crazy, "caca" in spanish makes way more sense for poop. Coco? Wtf, coco is a fruit. It's a coconut. At least now I know if I ever go to Brazil or Portugal, I can't go around saying that I want coco in my mouth.
adding the hiragana spelling since they already added the kanji to the course.
かのじょの おねえさんは こうこうせい です
の is being used as a particle to show that they are not the subject (は) of the sentence.
Let me get this straight. 高校 (こうこう) is an abbreviation of 高学校 (こうがっこう), right? Is the latter ever actually used, or is it an obligatory contraction?
Secondly, presumably because the 学 is left out, a student of this school is 高校生 (こうこうせい), rather than 高学生 (こうがくせい) along the lines of 中学生. Seeing as my predictive text won't write it, I'm assuming 高学生 isn't a proper word?
I believe 高校 is actually an abbreviation of 高等学校 (こうとうがっこう), and it's primarily used on official documents where one needs to write the name of a high school, like certificates, letter heads, etc. Don't ask me why it's different from the others f(^_^;
That's correct, "high school student" is 高校生. Again, I'm not sure what the reasoning behind this is, but my dictionary tells me that 高学年 is a word which refers to the upper year/grade levels within a primary/elementary school, for some reason :/ so 高学生 could possibly refer to students in those year levels. A quick search online seems to tell me that the phrase 高学生 isn't used at all though.
Would someone break this scentance down into parts for me please? Is the first part equivalent to a possessive "her"?
かのじょ = "she", "her"
の = possessive particle (AのB = "B of A" or "A's B")
おねえさん = "older sister"
は = topic particle
高校生 (こうこうせい) = high school student
です = "is/am/are"
Putting it back together, the topic particle は denotes 「かのじょのおねえさん」as the topic. This can be thought of as "the older sister of her (demonstrative)" or "her (possessive) older sister". We have です as the verb, so we are stating that the topic is a high school student.
"As for (= は) her (= かのじょの) older sister (= おねえさん), she is (= です) a high school student (= 高校生)" = "Her older sister is a high school student."
In polite Japanese the お is necessary whenever you are talking about someone else's family members.
what is the difference here, formality, dialect or something else?
You're missing a は, but 高等学生 isn't a commonly accepted phrase in Japanese. The full, formal version of "high school" is 高等学校 which is then abbreviated to 高校, in order to avoid confusion with 高等専門学校 (technical college, often shortened to 高専). Thus, a "high school student" is 高等学校の生徒 or 高校の生徒, which is then abbreviated to 高校生. According to a Japanese person's answer here, 高等学校生 is probably not considered normal because it's kind of a half-assed abbreviation.
姉（あね）is typically used for your own family. お姉さん（おねえさん）is more formal and used for other people's family members.
Also, おねえさん can be used to refer to a young lady who you don't know the name of.
So, is it more polite? Yes. Simply? No :/
Why は and not が? I thought to use が since introducing the high school student was new information.
The older sister is the main topic of the sentence, so は is more appropriate.
The second "の" signifies ownership. So, "かのじょの" means "her". Correct me if I'm wrong.
Correct. The の in かのじょ is only phonetic for 彼女, it does not indicate ownership.
います is used to tell someone that something exists. です is used to tell someone that something is something else.
Here, we are saying that "Her older sister" is also "a high school student". We aren't trying to tell anyone that "her older sister" exists, or "a high school student" exists; whether they exist or not is irrelevant to the information we are trying to convey.
いますmeans to exist, to have, same as あります except います is for people and living/animate things and あります is for inanimate stuff. So using いますwould end up meaning more like her high school student older sister exists, or like she has a high school student older sister. I am pretty sure that sentence can be made, but its not quite the same as the original.
Desu (Japanese keyboard is annoying on my phone) is like to be, is, am, are. So desu is the more correct translation.
I suppose you could say it either way, as far as it being grammatically correct. I'm not sure actually, grammar is hard, lol.
You can say かのじょのおねえさんは高校生です or かのじょのおねえさんは高校生(が)います, but while they are both grammatically correct, they mean drastically different things, so you can't "say it either way". In this case, です isn't "more correct"; it is correct.
Also, using います would mean "Her older sister has high school students", not as you suggested anything to do with "a high school student older sister". In more natural English, I presume you mean "a high school-aged older sister", which would require the phrase 高校生のおねえさん, or かのじょの高校生のおねえさん for "her high school-aged older sister".