It been said by Hermione that there are all kinds of enchantments around Hogwarts to stop outside people and forces from getting in. If this is true, then how in PS were Charlie's friends able to fly up to the tower to get Norbert? Shouldn't the enchantment have prevented them from getting inside the grounds? In the sixth book Dumbledore had to lower the enchantment for him and Harry to fly through when coming back from the lake. Was the "no fly" enchantment something that was added when Voldemort came back, or is it possible Dumbledore knew what was happening that night in PS?
This is an excellent question with no canonical answer - that is, Rowling, to my knowledge, never stated an answer to this, and I've had a look through the Q and A as well - This means that you've already answered your own question as much as I could: speculation. Dumbledore's certainly clever, and he might have known Hagrid had Norbert and there's little reason that he would stop Hagrid. Having the dragon's illegal, but so is having his wand still, doing magic and breeding new creatures. It's also possible that Dumbledore was told about it by Mcgonnagal, just after she caught Malfoy and just before Charlie's friends came.... however: Charlie went to Hogwarts so he would have known about protections, and thus in his opinion no one could fly in, regardless if Dumbledore knew. As for the protections being added after that year, this couldn't be possible as Hogwarts: A History talks about the protections... Really, I don't think there is a reason, it seems like it was just something that had to happen for plot that the author didn't really think about - there are a few little things like this in the first book (such as whether or not the mirror of erised always contained the stone - in which case it spent most the year completely unprotected by the Chessmen etc - or if it was just put in the mirror near the end of the book - in which case why wasn't it put in from the beginning - and the fact that 3 1st years got past all the enchantments that are supposed to stop one of the most powerful wizards in the world). So to conclude, there are guesses, but I can't think of any that would make sense; perhaps Rowling just didn't think about.
An end note: In the film's they solved this problem, in that Dumbledore sent Norbert away to Romania, and Harry and the others not being involved at all.
The rules governing the security of Hogwarts’ boundaries seem fairly loose before Voldemort’s return. You’ve cited the example of Charlie and the dragon; here are some others:
- In Chamber of Secrets, Harry and Ron cross the Hogwarts boundaries in the flying Ford Anglia, directly into the Whomping Willow.
- In Prisoner of Azkaban, Sirius escapes from Azkaban and walks straight into the Hogwarts grounds (albeit transformed into a dog). Later in the book, he flies away with Buckbeak without any apparent hindrance. (And despite the heightened security from the Dementor presence.)
- In Goblet of Fire, Barty Crouch wanders onto the grounds through the Forbidden Forest.
- In Order of the Phoenix, both the Weasley twins and members of the DA leave the castle by unofficial means.
- It’s worth mentioning that Dumbledore did this several times, in Order of the Phoenix and Half-Blood Prince, but Dumbledore’s skill is considerable, so that’s somewhat different.
I think the number of incidents where the boundaries were apparently breached suggests that this wasn’t a one-off. Dumbledore definitely didn’t have specific knowledge of some of these, so we still need an explanation without his helping hand in the general case, if not Charlie’s dragon.
I don’t have any details about your specific incident, but I’m going to discuss the general practice of people entering and leaving the grounds for “unofficial” business.
Once Voldemort openly returns, the castle defences are fortified, but we aren’t really given details. I’d be surprised if Dumbledore didn’t add some defences after Goblet of Fire, even if they weren’t Ministry sanctioned, then added more after Death Eaters successfully infiltrated the Ministry.
If I had to speculate, I’d guess that the defences are somewhat permeable. We know that they keep out Muggles (see Muggle-repelling charms, Hogwarts: A History, and so on), and people who wish to enter the castle by stealth (apparition). If there aren’t active hostilities, this seems like a reasonable security protocol. The restriction on Muggles is obvious, and forcing Dark entities to come via the front door is a major hindrance.
I’d go further and say that there are legitimate reasons to have permeable boundaries. If a witch or wizard comes to Hogwarts seeking sanctuary, then letting them in isn’t unreasonable. Or parents who come to see their child in an emergency. Or Ministry officials doing spot checks. Or owls!
I don’t think it’s feasible to validate and check in every visitor, especially if lowering the boundaries needs somebody of Dumbledore or McGonagall’s level of skill. If you’re a magician who walks across the boundary, you get in. Trying to sneak in gets you booted out.
The fear of Dumbledore is probably enough to keep most people with dubious motives away: if Voldemort never tried to attack the castle in the first war, what chance would they stand? (Plus, a common defence for crimes in the first war — the Imperius curse — doesn’t apply if you attack Hogwarts between the two wars.)
Once Voldemort is active again, the shutters have to come down. Rather than assuming that anybody who enters is a legitimate visitor, you have to treat everybody as potentially hostile.
As for why Fred, George and the DA can still leave? You’re trying to keep Dark entities out, not shut them in. Or maybe arrogance: if you truly believe that your defences are infallible, then anybody inside your barriers is obviously “nice”. Why would you want to stop them leaving? *cough* Half-Blood Prince *cough*
In summary, I would speculate that:
Before Voldemort’s return to strength, Hogwarts operated an “innocent until proven guilty” policy to visitors. Direct entry was openly permitted, but stealth was restricted. Once Voldemort returned, a policy of “guilty until proven innocent” was adopted.
In Philosopher's Stone on through Half-Blood Prince, the protections on Hogwarts were standard. Not only was Charlie's friends able to fly in, but in Order of the Phoenix, Fred and George were able to fly from Hogwarts.
After Voldemort's return was admitted by the Ministry, and his priority might have been attacking Harry and endangering Hogwarts' students, protections on Hogwarts were increased.
In Half-Blood Prince, on page 159, Tonks in taking Harry to Hogwarts from the train.
"Alohomora!" he said confidently, pointing his wand at the padlock, but nothing happened.
"That won't work on these," said Tonks. "Dumbledore bewitched them himself." Harry looked around.
"I could climb a wall," he suggested.
"No, you couldn't," said Tonks flatly. "Anti-intruder jinxes on all of them. Security's been tightened a hundredfold this summer."*
Later, during Dumbledore's address to the school he mentions additional protections (page 168):
"...The castle's magical fortifications have been strengthened over the summer, we are protected in new and more powerful ways...I urge you, therefore, to abide by any security restrictions that your teacher might impose upon you..."