Is the 'den' at the end of oradakiler ablative? Is there something I have missed about comparisons in Turkish?
It is the ablative :D If you want to say "X is more adjective than Y" in Turkish you say "X Y'dan (daha) *adjective)" :)
Does it sound better to include the plural suffix -ler in both words? Is it okay with just "oradakiden" here?
It is not okay with just "oradakinden" here, that would mean there is just 1 student over there
The software says "ordakilerden" but the correct answer is "oradakilerden". Is the former also correct? Is it a word?
"orada" is often pronounced as "orda." It is very common but isn't careful pronunciation (it would be bad to do this in a speech or during a presentation." In spelling, it is wrong.