"They go to high school by bike, or Sarah takes them there."
Translation:Elles vont au lycée à vélo ou Sarah les y amène.
"Ils vont au lycée à vélo ou Sarah les y amène." is accepted. Throughout this course, Duo accepts masculine or feminine for things like this (sometimes singular or plural, too) and many other things depending on whether there is something in the sentence or pronunciation (on spoken exercises) that demands one form instead of another (or limited selection of tiles). Duo goes out of its way to be inclusive, though its ability to include all national forms of English words, while exemplary, sometimes falls short. Really, at this point in the French course this should be obvious. Most of the time something else is wrong in the sentence. I do that repeatedly. I got this one wrong the first time, but it was for "Ils va", not "Ils".
In general terms emmener is to take someone somewhere (whether you stay with them or not - you would use it for dropping someone off at the railway station, for example.). Amener is to bring someone somewhere. But I think the distinction between the two in French is as flexible as it is in English and Duolingo is not consistent in its model answers.