an above us
and* above us?
I like the first sentence, though I'm not 100% it's grammatically correct.
since he had borrowed from the beginning a set of keys
This wording is a bit confusing.
One of those we asked for help to other guy friends and we had to take off the mosquito net using the clasp in our watches and then enter through the window
More confusing wording.
having what heard like an important chat with her friend,
having what sounded like?
having nowhere else to go
Strange wording. Maybe "with nowhere else to go"?
"Hey" I said "How did you solve the problem one of the exam today?"
"Hey," I said. "How did you solve..." (Missing punctuation)
Everyone had solved it but all did it differently.
Strange wording, and you need a comma before "but."
We called the attention of other guys that passed us, recognized the drawing, and asked from above: "what are you doing?"
Awkward sentence. "We caught the attention, and they recognized the drawing" maybe? Also, capitalize the first letter inside quotation marks.
"We are deciding which solution is the prettiest" we replied.
"We are deciding which solution is the prettiest," we replied.
You may be wandering
wondering*
, my case, are invited to four, ten-day long math sessions on different parts of the country.
In my case?
We call them: The National Trainings.
Colon unnecessary.
That I was famous for solving a combinatory problem using a polygon (I even won a nickname). Or that I once stole the football from a guy than runs like a gazelle (he stole it back, but it's still the biggest achievement of my brief soccer career). Or that I stopped being the same after my friend moved a rare Magic card closer to the window so I could see it, and moonlight fell on it and surrounded it and brought it alive (the lights were off because my roomies were sleeping).
Sentence fragment. Maybe it's a stylistic choice? I don't know.
Math was from the very beginning a place I loved to be at, but sitting on the floor that night, deprived of access to our room, I realized that it was no longer a place I visited by myself.
I'd tweak the first part of the sentence because ending with "at" sounds strange. I'm also not 100% sure what you mean here.
Overall, the essay has heart, but there were some grammatical errors that need to be fixed. Also, I feel like I could hear more about what you love about math or The National Trainings.
Good luck!