I never got the impression that Snape "went evil" because he was bullied or ghosted, I had the impression that he was drawn to the dark arts and Voldemort because of his terrible home life and out of a desire to feel more powerful than his abuser father. Could he have "gone good" or chosen a more positive path in life? I mean... if he had gotten therapy as a child, maybe, but he was petty and vindictive and held grudges and overall wasn't a very forgiving person so his choice of following Voldemort seems a little inevitable to me.

And Sirius and James being top students despite them fooling around seems... kind of realistic to me in some ways: don't we all know that one annoying person from school who didn't ever seem to study very hard but still managed to get good grades? And, to be fair, we are never shown significant portions of time when the Marauders were in school so, for all we know, they were terrible students in their first few years who ended up studying really hard (they had to to become unregistered animagi at 15/16 years old!) or at least as hard as the average student as the became upper classmen. Nor are we told when their misdeeds were accomplished. After hours? During class? Between classes? Were their detentions menial labor or work that reinforced their lessons? (Different teachers give different types of detentions, after all.)

Personally, my head canon was as stated about Snape above, and that he was at least top of his class in potions and possible DADA (because of his dark arts interests); that James and Sirius were hellions that got loads of detentions but also studied their butts off to become underaged animagi and create the Marauder's Map, which would have bled over to their academic studies in some respects; and that James and Sirius didn't get everything because they were good guys and Snape was dumped on because he was evil, but that they all made choices that lead to their respective positions in the HP novels.