Would adding paragraph about the motivation provided by someone who doubted me to my final year project acknowledgment be unprofessional? In my final year project, and since it will be my last postgraduate year, I want to add a taste of truth in my Acknowledgment, where I think it's the personal part of the project, where you are allowed to express yourself.

After thanking my professors, supervisors, parents in a very formal and professional way, I am eager to add this paragraph at the end. 

I had a period where I was the underdog in the high school, I was treated badly, and I lost motivation to study because I wasn't encouraged, all my work and even good results in exams were not even appreciated. I was called the stupid dude, the person who thinks he knows but knows nothing, and ... and all of the this was by my math teacher, not my colleagues.

After that I was accepted in one of the top universities in Lebanon, in one of the hardest entrance exams, with an acceptance rate of not more than 15%. And after entering the university I was a top student. 

For that I have written in the last paragraph of my acknowledgment: 


  Since it’s the end of the years I spent in the Lebanese
  University-Faculty of Engineering, I’m glad to thank the math teacher
  who once told me: "you will never pass the entrance exam of the
  Lebanese University-Faculty of engineering, my nephew failed surely
  you will fail ". Thank you dude, you were my motive, and proving you
  wrong was one of my goals.


Would this be unprofessional?
I would refrain from adding this to the written/electronic version of the project report. It's very personal and uses colloquial language that should be avoided in a formal report, even in the acknowledgement section.

Having said that, you may still give to that particular teacher a copy with your personal hand-written dedication.

Btw, great example of negative motivation (or what it's called): a negative comment with positive result.