The flaw in the plan

A meeting was taking place at the Malfoy Manor. The dark lord and his death eaters were discussing a very important plan. So important that only a few death eaters were attending.

"I need someone inside Hogwarts." Voldemort said, looking at the people sitting on the table.

The four death eaters that were there looked at him surprised.

"But, my lord, Severus Snape is there." A woman said.

"Yes, Narcissa. But you didn't let me finish." He replied, looking at her dangerously. "I need someone in Hogwarts that can be near Potter. That can be friends with him."

The man next to Narcissa shifted uncomfortably.
"Certainly, my lord, you don't mean my son Draco? They hate each other, my lord."

"You didn't let me finish as well, Lucius." Voldemort replied, a dangerous spark in his eyes. "I need one of you to drink polyjuice potion and get in the castle, pretending to be a student."

The death eaters remained quiet.
"Oh, no one has the courage to do this?" The dark lord asked, looking mockingly at the four people there.

"But, my lord, will we enter as first years? I don't really think it will work. The boy is going to the fifth year now. We would need someone that's, at least, in the fourth year". The other man in the room said, afraid for his life for giving his opinion.

"Yes, Rodolphus, I agree with you. And I, as you should know, have already thought about that. I think that a seventh year student would be appropriate." Voldemort answered, caressing his snake. "A girl to seduce Potter. Be his girlfriend. That way he won't have any doubt to go wherever she wants to go."

The other woman in the room smiled.
"That's a brilliant plan, my lord, as always."

"Yes, Bellatrix, I am brilliant." Voldemort said, looking at Nagini who was staying quietly at the corner of the room. "So, who wants to carry out my plan?"

Both men looked a bit disgusted at the thought of seducing another man and Narcissa was not very convinced by that plan.

"I will do it, my lord, with your permission." Bellatrix said, still smiling.

"Bella, I knew I could count on you." Voldemort replied, an evil smile on his lips. "I have the polyjuice potion ready, a stock for the whole year. I have already killed the girl you're going to replace. Her name is Lucy Prentiss. She is from Gryffindor. As you know, tomorrow you have to go to the train. Be there on time. Go, the meeting is over. And you can't tell this to anyone. This plan cannot fail."

So they all left. Bellatrix went to her room and found a bag with all the girl's belongings and the polyjuice potion stocked in small vials. She laid on her bed and slept. She would have a long year.