wouldn't it be "på finska"? or is this sentence trying to imply something else? the english correct answer isn't very clear either.
På finska means that she’s going to teach using Finnish as the language of instruction. I finska means she’s teaching the Finnish language.
Does "Shall you teach in Finnish" make sense? I'm mentally translating "ska" as "shall" because most sentences don't allow translating it as "will".
It does, primarily in British English, but we refrain from using "shall" and "will" synonymously since (especially) American English mostly does not do this. We find that not accepting it is unkind to a minority of our users, and accepting it confuses a much larger majority.
That's fair, though "shall" seems to be a valid translation in other sentences. (Accepted as correct in DuoLingo, I mean.) I'll try to remember "is/are going to" as a more accurate translation.
'Shall' should always be accepted for the first person (I and we), because that is the traditional British usage (used to mean will), but not for the other persons like 'you, 'she', 'they' etc because for those pronouns, will would be used to mean will even in traditional British grammar.
No, lära can also mean "teach" when used transitively. For instance: jag lär er finska = I teach you Finnish.