Ron returned after visiting his brother, Bill, who had been recently attacked by a new magical virus due to being bitten by a werewolf, Fenrir Greyback, in his not-so-ready form. He was depressed because his owl, Pigwidgeon was malting and leaving feathers everywhere. After Hermione explained what had happened with Snape, Ron gladly agreed to accompany them both, just to shake his fist at the greasy-haired criminal. Harry didn't need any more assurance than his two best friends. Luckily Hermione hadn't mentioned his little 'date' with Cho, because if Ron found out, then Ginny would find out and that would be a disaster.
The three of them were on their way to Defence Against the Dark Arts, when Ernie Macmillan stopped Hermione to tell her that the Prefects and Head Boy and Girl were wanted by Professor McGonagall for an emergency meeting regarding You-Know-Who. He also said that Harry was called up as well, leaving Ron behind. This didn't put the young Weasley in a particularly fine mood.
Harry followed Ernie as he led Hermione and himself to the Headmaster's office. When they finally arrived, Ernie murmured something along the lines of, 'Mrs. Norris is a sneaky pussy-cat,' to the stone gargoyle and it admitted them to the very familiar stone stairwell that led to Mrs. McGonagall's new office.
When they reached the top of the long winding stairs, Colin Creevey pulled them inside with an alarmed look on his face. Apparently Mrs. McGonagall had been explaining that new charms and curses are needed to be placed on the walls and gates of the castle as there had been numerous sightings of Death Eaters around Oxford.
She began by saying, 'We are going to need the help of the teachers and the older students to protect the castle as the Ministry of Magic has its hands full with protecting the muggles and local magic folk.' She continued in a rather distasteful tone, 'I have called upon Professor Molarch, the new Defence Against the Dark Arts teacher, to place additional curses and charms on the walls near the lake and the Forbidden Forest. There is also the problem of patrolling the hallways after hours. I have three teachers, Professors Molarch, Slughorn and myself, but I would find it helpful if some of you more responsible students began patrolling the floors of which your house common rooms are situated.'
She continued to ramble on about calling in some kelpies (violent, amphibious shape shifters) to protect the mer-people and the underwater caves, before finishing with a chat to Ernie, Hermione and himself.
For a few seconds she sat there massaging her temples before speaking in a hushed and wearied voice. 'This problem of ours is going to get worse if we don't use the knowledge that we already have of our opponent. I will begin by asking you, Harry, not to go looking for Draco Malfoy, because revenge isn't worth the lives of everyone here at Hogwarts. Secondly, I will ask you to work with Professor Molarch to see if you can create a few more anti-magic curses and charms. We maybe able to protect ourselves with new and improved magic against our foes. That is all I am going to ask of you. Do you agree to comply to my requests?'
Harry nodded and was granted leave from the meeting.