Percy Jackson Teaches at Hogwarts (Percy Jackson and Harry Potter crossover)

The first few days

Percy POV

The first few days were a little boring, I give them the page number, they read, they try the spell. I really missed my Wise Girl.
On the third day, I got a surprise, an IM from Annabeth!
"Hey wise girl, how's it going?"
She looked really happy.
"Percy, you know how I had that doctors visit yesterday?"
"What about it?"
"It's TWINS!"
I tripped over my desk, breaking a few things, and creating a loud sound.
"I hear your class coming, so I'm going to cut off here," she wispers, "see you!"
I lay on the floor for a moment in shock before, sure enough, my class came in.
"Everything ok Professor?" I hear someone call.
"Yeah," I say getting up, "Just tripped. And call me Percy."
The students all took their seats and I started lessons as planned. I was teaching the 7th year repeats. I was humming to myself, doing a little dance when I was writing in the board. One student apparently thought this was out of the ordinary, and cast a spell at me. The impact sent me flying into a wall. I figured that something was supposed to happen, so I just lay there.

Harry POV

I tried looking for something different that could be the reason he was the teacher. All we did was review the spells we learned before.
Nothing happened for three days, then, we were walking down the hall towards DADA, there was a loud crash. We all ran, to make sure nothing was wrong. Probably another Lockhart. I thought. He probably got a creature to practice with, and lost control of it.
We run in to see him lying on the floor by his desk. Hermione calls out, "Everything ok, Professor?"
He insists he's fine, correct her, and teaches his lesson. He was humming, which was kind of uncommon, because he was usually moody. His normal long sleeve shirt shifted, and ask the students saw a tattoo on his arm.
He's a death eater!
Apparently, someone else came to the same conclusion. Unfortunately, it was one of the kids who jumped at anything. He shouted the killing curse, and the impact knocked 'Percy' against the wall.
Don't get me wrong, his lessons were all boring review, but I didn't think he should have to die.
The kid who cursed him was shaking, but Hermione explained the reason behind the suspicion,
"If you think about it, it make almost perfect sense, a death eater with a polyjuice (sorry if thats wrong) potion. He has a sudden change of attitude after sounds of a fight, he has a tattoo, and in a week, most of the school will be gathered for his 'PTSD help' and where did he even get PTSD? I don't know about you, but I didn't see him during the battle. He is younger than us, and he has more experience with the dark arts." We all nodded, it made perfect sense!

"One problem with that," a voice from his desk said, "Well, actually, multiple." We all looked, and he was sitting at his desk perfectly fine! "My tattoo is from the military, and there was a war in the US as well. I am actually 21, so I am actually older than you."
"What about the crash? And how did you survive the killing curse?" I asked. I still doubted he was innocent, he survived the killing curse just fine.
"The crash was because my wife called me... I'm going to be a father of TWINS!!" He yelled, looking really happy, "And as for the curse," He got up and went to the door, "I can't die." He said "Class dissmissed." The door slammed as he left. Who was Percy Jackson?