LXX, didache, etc.
If they're learning NT Greek, it's presumably because they want to read (mark, learn, and inwardly digest) the GNT.
I am not a teacher, but from my perspective as a linguist, exegete and Bible translator I would like suggest that it is important to read secular Greek, if you want to understand the NT Greek. If the only texts of the epistle type the students read, are the ones in the NT, then it is easy to have tunnel vision. It is easy to misinterpret the rhetorical devices and even the grammar that Paul uses when there is no context of how those devices are used outside of the NT.
And even for the beginner student starting with narrative, I think it would be beneficial to start with secular Greek, like Xenofon. If a student reads Gospel stories and ponders for example the use of aorist or historical present, that pondering in the context of the NT can lead to wrong conclusions, because it is religious text and inspired holy text. If one encounters aorist and historical present in secular texts first, those potential wrong conclusions are excluded. That may result in inaccurate preaching, teaching and translation.
Just my two cents worth.