How did Draco apparate within Hogwarts?

There's no definitive canon answer in the film canon that I know of (including on any of the extra features on DH2 Blue-Ray, interviews, or the script).

However, in DH2 film, Death Eaters are able to Apparate around the castle during the battle (and Voldemort, when fighting Harry as well); meaning that the Anti-Apparation Jinx isn't in effect anymore.

Possible explanations are that:

1. Voldemort had managed to break the Anti-Disapparation spells when he broke the wards

2. Or they were sabotaged by Snape earlier on his orders, acting as Headmaster. As we can see from Half-Blood Prince movie script, Harry and Dumbledore Apparated at the Astronomy Tower, and Dumbledore told Harry he can open/close "windows" for apparation, as Headmaster:

   DUMBLEDORE: Take my arm.
   HARRY: Sir, I thought one couldn'ta apparate within Hogwarts.
   DUMBLEDORE: Being me has its privileges.

   EXT. ASTRONOMY TOWER - NIGHT (MOMENTS LATER)
   Harry and Dumbledore materialize on the rooftop ...
   ...
   Harry ... sees Dumbledore MUTTERING WEARILY as he GESTICULATES with his blackened hand.
   HARRY: Sir, are you... praying? DUMBLEDORE: (smiling faintly) No, Harry. I do not pray. I was merely closing a window -- the one that had allowed us to Apparate.

I think because of Dumbledore's death, all the enchantments he cast over the school were gone. If anyone recalls, in The Half-Blood Prince, when Dumbledore passed away the Petrificus Totalus he cast on Harry was released. I believe the Anti-Apparation Jinx was also lifted when Dumbledore died.

It’s a movie mistake. In the books, for example in the 6th, Harry and Dumbledore have to fly to Hogsmeade in brooms and then apparate from there because you simply can’t do it in Hogwarts. In the 7th book, there isn’t any mention of anyone apparating, in fact, the long duel and fight between Harry and Voldemort (where Voldemort apparates many times) that we see in the film, is actually a long chat in the Great Hall where Harry reveals Snape’s true loyalty, and then they have a brief duel where Voldemort’s curse rebounds and kills him.