It has been taken from a children's English study course.
"He looked at the children in front of him. He thought they were sneering at him as he stood there feeling foolish, so he ... (a suitable tense form of the verb "to smile" is to be inserted)... as he trudged to his desk and flung himself down in his chair."
In the test keys section is a one-valued answer adduced as the only right one. And it is "[he] didn't smile".
I clearly take in the point that with the verbs of durative meaning when attention is focused on the circumstances under which the action is performed the Past Continuous may be swapped for the Past Simple. And that is why the Past Simple has been brought in as an answer here. But I can't get my head over the Past Continuous to have been left out of the right answer options.
Considering the aforesaid, why has the Past Simple been made the only appropriate answer in: "...he didn't smile as he trudged...", whereas the Past Continuous appears to be correct (and it is, obviously) in: "He thought they were sneering at him as he stood...", which are very similar patterns, at least in my oppinion?
And would it be nevertheless correct to say: "...so he wasn't smiling as he trudged to his desk and flung himself down in his chair."?