Everything appeared to be going well for the Order, but everything quickly took a turn for the worse.... Dumbledore has been ousted from Hogwarts, and the Muggleborns have been forced to register with the Ministry. Minister Kirke has placed Bellatrix Black in charge of Hogwarts, and Dumbledore has been forced into hiding, to be arrested for hiding Muggleborns.
To save Muggleborns and those who support them, the Blackhorse Family has opened up their property to allow Dumbledore to teach, in secret, those that can no longer attend Hogwarts. A fair few are going with him, while the rest are forced to go back to Hogwarts, whether out of fear of the government, or loyalty to the Dark Lord.