minor subtleties.. "the girls are by the window" = "τα κορίτσια είναι κοντά στο παράθυρο" clarifying the proximity of the girls to the window, without addressing their visibility through the window. "τα κορίτσια είναι στο παράθυρο" = "the girls are at/in the window" implying that the girls are visible through the window without clarifying their relative distance to the window... "by" needs the word "κοντά"
Yes, I get your point and if was in a classroom the teacher would know very well that it was a slip up. But when there is a computer program grading the exercises you can see how that doesn't work.
Now, about "correcting" after the check, that would be a good feature. But I think hard to program. So l guess that until the computer program can be fine tuned we'll have to review before we send the answer. There are of course single word typos that are overlooked with a simple "You have a typo." message.