why don't I fight back

Dungeon and tower have different tiers, towers go from F - S tier and Dungeons go from S - ZZZ tier

Today at class we had to learn to fight but their is one thing bad about that, im basically a regular human just can't die, I can gain muscle and never gain weight and I even age slower, so I'm not the best at fighting, so I don't like doing it. 

The worse part is that since I'm immortal the teacher puts me against the strongest people in our class. 

The first 5 weeks of training I had to fight Joe (Air Blades) at first I thought I would just need to dodge from the front, but then I learn very harshly that that's not the case, the first round I ran at him as fast as I could, were all 15 so he must not be that strong, im 17 I just look 15 bit either way I'm faster than him, I should make it before he can use his power and them I felt a sharp pain on my right elbow

He still hasn't moved

I looked at my arm and it was gone, but strangely it didn't hurt much, so I kept my pursuit, he looked confused that I ignored my arm

He started to move his hand up and I suddenly couldn't feel anything, couldn't move, feel, 

I'm on the floor

My neck hurts 

I can't look down

I feel paralyzed 

I heard my teacher say that Joe won, but I'm still fine

Wait, my heads on the floor and my body is infront of me, did he cut my head off, that's cheating, my body started growing back, it takes about an hour, apparently it takes less time the older I am.

The more I fought Joe the more I figured out that I wouldn't win, ever, it git to the point where I would even move and my head would be off my body. 

There was ones that I got close but I didn't get a hit in, it was just a splash of blood.

I knowtest something that happens when I get my head cut off, my body gets more muscular.

I finally convinced the teacher to switch partners with someone she switch me to fight charlie(teleportion)

 I finally could get a few hits in, it felt to easy, I do t think he was trying, he would only teleport when he knew he would get hit, and wouldn't attack back.

A few weeks later they started another project called ex 5, I know what it is because they've done the same thing to me as a child, it's experimental, this is the fifth time their trying it, but for some reason they only brought me. They told me to go in the white room at wait for them, so I went in and sat at this black table and chair and then I heard the vents turn on and it smelled so sweet, I didn't know what they wanted, but I felt sleepy and past out, when I woke up I was tied to a table with a piece of the asteroid above, I got a but scared, it felt hot some people walked in with white clothes and mask, they were talking to each other, it was about an hour till they pickup a hand saw and came close to me they said that they wanted to try something they wrote off the board till now because of my age, but now that I'm at a high age and can talk their going to try another experiment.

This mostly only happens to people who have powers that have never been reported before

They didn't bother putting me under any pain blockers before they started the cutting, it of course hurt but I didn't really mind it but then they lowered the asteroid piece and I started to fell weird, the pain was getting worse, they finally put the stone in my body and everything went black, I felt calm and safe, I felt their was something here but I couldn't figure out what, then like I was sucked out of a dream I woke up on the table, my body was fully recovered from the cutting. They let me off the table and allowed me to go back to my room.

They are not good at hiding things. On the way back I felt like I've been reborn many times, there was a janitor in the hall way pushing a basket completely full of something, I ask what he was doing, he looked at me Disgusted like I did something wrong, I've not a very mean person, if I could chill my entire life I would.

I asked him what those eyes are for. He stood there for a minute, started to grab something out of the basket and hesitantly he pulled out a arm that look like mine and said whispering bye, while waving it back and forth. I know that's my arm so I didn't care much, but that all I needed to know.