I suppose one could go back and look at popular style guides from the 1980s and 1990s and see if they endorsed it.
And your teacher would presumably have marked down Shakespeare for the same thing. If it was good enough for Chaucer, Shakespeare, and Austin, you'd think it would be good enough for your teacher, but we went through a particularly prescriptive period in the early to mid 20th century (though your teacher was maybe slightly behind the times even in the 80s).
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1. While in most languages, this class of verbs becomes apparent when the base noun is irregular or has special conjugation rules, in Greek,² it’s especially noticeable thanks to the fact that the aorist causes a morphological change to the beginning of the verb as well as the end. Except for these verbs, the morphological change ends up happening in the middle of the verb.
2. I don’t know modern Greek beyond what I can discern from my classical Greek knowledge, so I don’t know if modern Greek has retained this feature.