Translation:Who sits down by the student in the school?
Then I have problem with the English phrase here. What does "sit down by" mean?
The Hungarian phrase "ki ül le a diákhoz" is already a bit strange. I imagine a student sitting at a table, and someone comes and sits down to the same table. (If that is what you mean as "sits down by" then fine for me :)
In English, there is no "sit down to" an "object". You can sit down "by" something, "near" something, "next to" something, you can sit down "with" something, etc. You can sit down "to" a concept - a meeting, a discussion, a meal.
So here, yes, the English would be "sit down by" or "sit down with".
That would be the postposition -hoz in this case "a diákhoz".
Not sure why you think this sentence should never have been used in this lesson, as it fits the subject of the lesson just fine.
For all its limited vocabulary, Hungarian is not an easy language for an English speaker. It's my 4th language (outside of having used Duolingo) and for all the ways each language is supposed to help make subsequent ones easier, Hungarian is still tricky. It's going to require patience.