Please kindly avoid spoilers for book 6 & 7.
In the goblet of fire, when Harry & Krum enter the forest to "talk", they meet Barty Crouch sr. who was under Voldemort's imperious curse.
So how did he enter the forest? Can anyone enter the forest and thereby entering into Hogwarts?
I know the forest isn't the best route to take to the school ground but if an imperialised Crouch could do it without even Dumbledore knowing, who couldn't?