Professor McGonagall woke up in the middle of the night and said, "I have a good idea." Then she drank a polyjuice potion containing Osama bin Laden's eyelash, which turned her into a perfect lookalike of him. Then she flew to Abbottabad, Pakistan on a broomstick.
McGonagall finds Osama bin Laden's compound on MapQuest and goes there. When Osama bin Laden sees a duplicate of himself standing at his front door, he says, "You're me. Let's get married." So Osama and McGonagall (who has morphed into Osama) get married the next day.
Professor McGonagall continues drinking the Osama bin Laden polyjuice potion every day, so she always looks like him. When the people of Abbottabad see Professor McGonagall walking around they say, "There goes either Osama bin Laden or his wife, who is also Osama bin Laden."
Professor McGonagall and Osama bin Laden are completely indistinguishable from one another. Professor McGonagall starts calling herself Osama bin Laden and calling the real Osama bin Laden "Osama bin Laden, Jr.” It's impossible to tell which is which.
While she's living her life as Osama, Professor McGonagall keeps doing weird witch things. She buys like 50 owls a month and kills them for spells. She tosses the carcasses into a big barrel in the yard labeled "Osama bin Laden's Dead Owls." She also helps run al-Qaeda.
Things continue like this for about two years, but then SEAL Team 6 notices the barrel of owl carcasses labeled "Osama bin Laden's Dead Owls" outside the compound and figures this is probably where Osama bin Laden is hiding.
SEAL Team 6 storms into the compound and locates someone who looks exactly like Osama bin Laden. They shoot him in the head, spraying his brains all over the wall, and as soon as this happens the man they thought was Osama turns back into McGonagall with her head blown open.
SEAL Team 6 doesn't notice. They think they killed bin Laden. They call Barack Obama and tell him about it while he's in the bath. He's hooting and hollering in the tub and joyously splashing water, unaware that "bin Laden" is actually a teacher from a magical wizard school.
Meanwhile, the real bin Laden escapes the compound on the back of a hippogriff and flies to Hogwarts. He walks in and says, "I'm Osama bin Laden and I need a job." The faculty immediately says yes. The former sex ed teacher was recently eaten by a troll so they need a replacement.
Osama bin Laden starts teaching sex ed the next day. He teaches kids about puberty and safe sex. He actually does a pretty good job. Sex ed becomes the most popular class at Hogwarts. Harry Potter's son Severus learns what a dental dam is from Osama bin Laden.
Whenever any student asks where Professor McGonagall is, the teachers tell them that she got fired for turning a kid into a Honda Civic.
SEAL Team Six dumps Professor McGonagall's carcass into the ocean. Because her head was more or less blown off by a gun, they don't really notice that Osama bin Laden has turned into an elderly witch.
To this day, bin Laden is the sex ed teacher at Hogwarts. He teaches young witches and wizards all about their changing bodies. In 2017 he won the Albus Dumbledore Award for excellence in teaching, and the Hogwarts Library was renamed "The Osama bin Laden Library" in his honor.