What happens if you try to apparate into Hogwarts?

Question

We learn that using apparition into and out of Hogwarts is not possible.
When Dumbledore is using apparition with Harry when they are going to find the locket, Harry asks him how can he do that, and he says that it's something a headmaster can do.

When an instructor from the Ministry Of Magic arrives at Hogwarts to teach apparition to the students, Dumbledore says that it is now possible to apparate inside the dining hall, and warns them that they should not try to apparate outside of the room, and makes it sound dangerous.

Does it ever say what will happen if someone tries to apparate into Hogwarts?

Answer 1

In Chapter Eighteen of Half-Blood Prince when the Ministry instructor explained that the anti-apparition enchantment had been lifted from the Great Hall for them to practice, he said:

May I emphasize that you will not be able to Apparate outside the walls of this Hall, and that you would be unwise to try.

The implication here seems to be that if you do try to Apparate against the enchantment, some unspecified bad thing will happen to you.

However, in the two instances in which we actually see people trying to Apparate against enchantments, nothing actually seems to happen to them. In Chapter Twenty-Three of Deathly Hallows:

Ron was now trying to Disapparate without a wand.

"There's no way out, Ron," said Luna, watching his fruitless efforts.

In Chapter Twenty-Eight of Deathly Hallows:

The air through which they needed to move seemed to have become solid: They could not Disapparate; the Death Eaters had cast their charms well.

While neither of these two events occur at Hogwarts, we have no reason to assume that the anti-apparition charms at Hogwarts are fundamentally different from anti-apparition charms used elsewhere. Indeed, in Chapter Four of Half-Blood Prince Dumbledore mentions Hogwarts as an example of the anti-apparition charms placed on most Wizarding dwellings, implying that it is the same enchantment:

In any case, most Wizarding dwellings are magically protected from unwanted Apparators. At Hogwarts, for instance —"

"— you can't Apparate anywhere inside the buildings or grounds," said Harry quickly. "Hermione Granger told me."

It would therefore seem that nothing happens when you try to Apparate where there are anti-apparition enchantments in place. The Ministry instructor may have simply been trying to scare them into behaving.

(Montague did nearly die trying to Apparate out of the Vanishing Cabinet, but that was not necessarily related to the fact that Hogwarts had an anti-apparition enchantment.)

Answer 2

It can cause disastrous effects.

In the books, the ministry instructor warns students not to even try:

May I emphasize that you will not be able to Apparate outside the walls of this Hall, and that you would be unwise to try.

As an instructor, he would presumably know what he's talking about.

Furthermore, in Pottermore's Hogwarts: An Incomplete and Unreliable Guide it says:

As we know from early historical accounts, and from the evidence of early woodcuts and engravings, Hogwarts students used to arrive at school in any manner that caught their fancy. Some rode broomsticks (a difficult feat when carrying trunks and pets); others commandeered enchanted carts and, later, carriages; some attempted to Apparate (often with disastrous effects, as the castle and grounds have always been protected with Anti-Apparition Charms); others rode a variety of magical creatures.

So we see that there are times where "disastrous effects" happen when apparating into a protected zone.

In the books, as Alex points out, nothing happens. But that just means that those times nothing happened. (as the Hogwarts Guide points out, disastrous effects "often" happen, not always.)

(Furthermore, it's possible that the anti-apparation charm in Hogwarts in unique, but that's unlikely, as Dumbledore explicitly compares private anti-apparation charms to Hogwarts' in Half-Blood Prince.)