One of the more interesting points about Minerva’s rather harsh punishment of Harry, Hermione, and Neville in their first year is that she was punishing them for the wrong thing.
Which of course means that their punishment couldn’t teach them an appropriate lesson. Even had Dumbledore at the Ending Feast not countermanded it, you don’t learn from suffering consequences if the consequences are for something you DIDN’T do, rather than for what you did. The only lesson Draco, Harry and Hermione really learned that night was-well, there were two. Don’t get caught, and don’t trust Minerva to even try to get to the bottom of things before she starts flinging about rewards and punishments. (Reinforced for Harry later that year when she wouldn’t listen to him about the Stone, and in fact a recap of her failure to investigate why he was in the air against orders during his first broom lesson.)
But let’s think about what Minerva DID punish them for. Draco got 20 points taken plus a detention for being out of bed and (she thought) lying. She never allowed him to present his evidence that he was telling the truth, and so when she caught Harry and Hermione she did not punish them for being engaged in dragon-smuggling. But nor did she punish them merely for being out after curfew.
She took 150 points from Gryffindor and assigned all four children involved a detention that she expected to be terrifying for them and should have anticipated might be dangerous (though one hopes she didn’t realize Hagrid would split the group and let children wander the Forbidden Forest at night with a unicorn-killer presumed on the loose with no escort).
But this was not a punishment for dragon-smuggling, a crime that endangered themselves and other student. Harry and Hermione were given this draconian punishment for, Minerva thought, lying to Draco to try to induce him to break school rules.
Neville and Draco were punished (Neville as harshly as his peers) for being the H’s patsies, falling for their lies and breaking curfew themselves in response.
Now, we don’t know what the staff knew about the Malfoy-Potter duel. If Minerva knew perfectly well that Malfoy had lured Potter and Weasley into rule-breaking under false pretenses of dueling, only Dumbledore’s rules meant that none of the miscreants involved could be punished (none of them having been caught in the act), then it might have seemed reasonable to her that Potter and his friends might set Malfoy up the same way as revenge. Tit for tat.
So her hastily assuming she knew what was really going on without asking the kids themselves might have SOME excuse.
But her response seems really a little over the top for the supposed offense. If Draco gets 20 points plus detention for, she thought breaking curfew and lying-not rescinded when she learned he was, she thought, a victim of others’ lies rather than a liar himself-why take a full 150 points from Gryffindor? Including 50 from the boy she believes wasn’t lying, just sticking by his peers while they did something wrong?
Well. Look what she thought was going on. A Potter and his mate induce a Slytherin rival to break school rules and sneak around after curfew. In an attempt to catch them redhanded in an expulsion-worthy offense. And a second-tier friend, a sometime hanger-on, is involved somehow as a patsy/victim.
Echoes, anyone? What did Minerva know about the Prank?
If she thought Harry and his friends were replicating his father’s Prank-and believed that the Prank had endangered the lives of both James and Severus-then nipping that behavior in the bud, now, before it escalated to a nearly lethal confrontation, was worth giving away the House Cup. Neville’s sneaking out to try to warn them, then, was equated to Remus’s continued spinelessness in the face of his friends’ determined and dangerous rule-breaking.
And giving ALL the children involved a frightening, and maybe dangerous, detention was a way of trying to underline the true peril of the path they are all set on.
Seems that Severus is not the only one to see Harry as a mediocre-magic copy of James.