"Her older sister is a high school student."
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.
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."
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".
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.
For about 3 previous exercises on this lesson it accepts 彼の兄は for an answer, yet for this 彼女の姉は is incorrect. I understand why everyone is saying the honorific form is correct, but the lesson needs to be modified to remain consistent. It's very confusing.
Marked correct previously: 彼の兄は中学生です。 彼の兄は小学五生です。
I answered with 「彼女はお姉さんが高校生です」. I can understand why this wouldn't be quite right for the requested english sentence "Her older sister is a high school student", but I am wondering if there's a way to phrase something like "as for her, her older sister is a high school student", perhaps in the context of "and her brother is a university student", etc
I use google IME so I will help you based on that. A lot of people use the microsoft IME but I found that for me it's usually harder to use because is missing too many things.
If I type こうこうせい I usually get the whole word 高校生 by just pressing space afterward. However, I can for example press space and before pressing enter I can press shift+left arrow and it will let me select the section of the word I want to change independently, in that case it's like typing two words at once こうこう・せい and I can change the kanji for both (Arrow keys for that). If you type せい alone, you will usually get 性 which is more common than 生 alone. So you can probably do the same but the other way around, you can type こうこうせい and press shift+right arrow. Or you can also just type 高校 and then せい and press space until you get the kanji you need.
In phones is something similar, but I think is easier if you type こうこうせい and then look for the right kanji there.
Some people also use the tab key for the autosuggestion and it locks them into particular options (they are based on your typing history), so be sure that you try space if you are only using tab.
If you are still having trouble after that, you can also add the word manually into your dictionary. I recommend these articles that explain these things for different platforms: