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In Goblet of Fire, Snape suspects Harry of stealing boomslang skin and gillyweed from his office:

‘Don’t lie to me,’ Snape hissed, his fathomless black eyes boring into Harry’s. ‘Boomslang skin. Gillyweed. Both come from my private stores, and I know who stole them.’ Harry stared back at Snape, determined not to blink, or to look guilty. In truth, he hadn’t stolen either of these things from Snape.

If Snape could perform Legilimency, why didn't he know it hadn't been Harry? In Half Blood Prince he reads Harry's mind in a similar fashion to find out where he learnt Sectumsempra.

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While it's a pure speculation, maybe Snape is just so sure that Harry stole that things that he thought he don't need Legilimency... In case of Sectumsempra, he wants to get additional information. –  Lukasz Mar 11 at 17:35
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Okay, I didn't re-read the Seectumsempra scene before making this comment, but I had always interpreted Snape putting two and two together re: Sectumsempra because he remembered himself writing the spell into his potions book back in the day and, after recognizing that particular spell had been used on Draco, that Harry must have come into possession of Snape's old book. Again, I did not re-read, but if Snape used Legilimency on Harry after Sectumsempra, I somehow missed that. –  Slytherincess Mar 11 at 19:41
    
There's some line about "Harry had never been able to block him" and "the image of his potions book floated to the top of his head". –  ike Mar 11 at 19:46
    
I added the quote that includes Snape using Legilimency to my answer. –  alexwlchan Mar 11 at 19:53
    
Well, in that quote he isn't blaming Harry. Maybe he needs Harry to out Neville before he can punish anyone... –  Izkata Mar 11 at 23:10

4 Answers 4

I don’t have canon evidence to back this up, but I think it probably comes down to a matter of legality and ethics.

We know that you can’t use Veritaserum on students. We know that the use of Veritaserum on students is regulated (thanks ike and Mac in the comments for correcting me). In a similar way, I’d expect Legilimency to be restricted, perhaps even banned.

Using it on a student is a clear violation of privacy, and abuses the relationship between teacher and student. If Snape was using Legilimency on students on a regular basis, somebody would eventually realise that he was doing it, and he’d probably get kicked out. You can think of plenty of situations where it might be useful (did the dog really eat little Johnny’s homework?), but not where it would be necessary.

In Muggle laws, we have innocent until proven guilty. This restricts police powers to search people (note IANAL). For example, UK police generally aren’t allowed to search somebody unless they have reasonable suspicion that a crime has been committed. I think this rule probably applies to Legilimency and Veritaserum.

If we look at the specific instance in HBP:

“I didn’t mean it to happen,” said Harry at once. His voice echoed in the cold, watery space. “I didn’t know what that spell did.”

But Snape ignored this. “Apparently I underestimated you, Potter,” he said quietly. “Who would have thought you knew such Dark Magic? Who taught you that spell?”

“I — read about it somewhere.” “Where?”

“It was — a library book,” Harry invented wildly. “I can’t remember what it was call —”

“Liar,” said Snape. Harry’s throat went dry. He knew what Snape was going to do and he had never been able to prevent it. …

The bathroom seemed to shimmer before his eyes; he struggled to block out all thought, but try as he might, the Half-Blood Prince's copy of Advanced Potion-Making swam hazily to the forefront of his mind.

And then he was staring at Snape again, in the midst of this wrecked, soaked bathroom. He stared into Snape's black eyes, hoping against hope that Snape had not seen what he feared, but –

Umbridge aside,1 this is the only instance I can recall where Legilimency or Veritaserum were used on a student outside Harry’s classes. Here, Harry has already admitted that he used Dark Magic, and Snape walked in on him red-handed: the guilt is clear. This is more than just a schoolboy prank: these are very dangerous (probably life-threatening) injuries. If it wasn’t Harry, this would probably be grounds for expulsion or prosecution.

As serious as stealing from a teacher is, it isn’t really on the same scale. To me, Snape telling Harry he “knows” who stole his ingredients looks more like an intimidation tactic than anything else.

1We know she planned to use the Cruciatus curse as a disciplinary measure, which is definitely illegal. She was also overenthusiastic with the Veritaserum, and used it in large quantities. I don’t think what she did was really legal.

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"We know that you can’t use Veritaserum on students." From where? Can you give a quote for that? –  ike Mar 11 at 16:43
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“It is Veritaserum — a Truth Potion so powerful that three drops would have you spilling your innermost secrets for this entire class to hear,” said Snape viciously. “Now, the use of this potion is con-trolled by very strict Ministry guidelines. But unless you watch your step, you might just find that my hand slips” — he shook the crystal bottle slightly — “right over your evening pumpkin juice. And then, Potter … then we’ll find out whether you’ve been in my office or not.” Harry Potter and Goblet of Fire. Not quite saying it's illegal ... but close. :) –  Mac Cooper Mar 11 at 17:28
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I was going to use Mac’s quote, or there’s a slightly more explicit line in the GoF film: “Three drops of this and You-Know-Who himself would spill out his darkest secrets. The use of it on a student is – regrettably – forbidden.” (1:49:40) –  alexwlchan Mar 11 at 18:45
    
But that's lower canon status. –  ike Mar 11 at 19:41
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The movies aren't canon, IMO. The book quote is a better defence. :) –  Slytherincess Mar 11 at 19:45

It is probably unethical for Snape to invade the mind of a student. He did it in Half-Blood Prince because Harry had injured another student (Malfoy). Without cause, he likely isn't allowed to read the minds of the students willy nilly.

Snape may not have personal ethics, but if he invades Harry's mind without justification, there might be repercussions for him as a teacher.

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That assumes Snape cares about ethics ... –  ike Mar 11 at 16:42
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You might like to finish the books before judging Snape's ethics. –  DJClayworth Mar 11 at 18:02
    
I have finished the books. See this for an analysis of Snape's ethics :) –  ike Mar 11 at 18:39

Because I don't think performing magic on students is allowed at hogwarts. Not without the permission of Dumbeldore at least.

Reference: Scuffle between Moody and McGonagall over turning Malfoy into a ferret.

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It's best to actually quote from the source for a reference, or at least give the book, chapter, page number, something. –  Meat Trademark Mar 12 at 6:23

It's possible that Snape knows a great deal more about what's going on. He really does know what took the Gilly Weed

Dobby took it for Harry, on his behalf .

Of course admitting this might have consequences for both Harry and Snape.

The further possibility is that the reason he didn't do anything is that he knows some of why Harry is in the tournament and has a vested interest in Harry surviving to the final trial.

Either of these mean that Snape has no use of Legilimency in this case.

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