Engineering School after Information DUT

With your average in math, it's a deal-breaker.
I finished my DUT with an average of 12.6, having dropped to 10 in math (while I was at least 12.5 throughout both years) with a favorable opinion for further studies, I wasn't accepted into the schools I wanted (Polytech'Lille, ENSIIE, ISTY).
In fact, even for a Bachelor's/Master's, it seems impossible for me...

You could possibly apply to SupInfo, there you have all your chances (they're not strict about grades...).
But for engineering schools (recognized by CTI) and Master's programs, it's dead (try for the Bachelor's anyway, you never know).

I knew that math could be a handicap, but not to this extent. My grades are solely the result of my lack of seriousness over these 2 years, and not at all due to my motivation to get into engineering school.

Are there any preparatory courses available after a bac +2 that can help improve my level?

Maybe the ATM preparatory classes... We'll see because even the preparatory classes are not guaranteed with your grades.

Maths grades are almost more important than those in computer science for engineering schools, it's quite impressive.. :/

Otherwise, tibobo_77's solution is good, but you must have taken the ATS preparatory course (not ATM), which is for bac+2. And given the grades, it seems quite unfeasible..
In any case, try to finish your year strong, and write an ultra convincing letter about the moment you decided to go to engineering school - addressed to a targeted school or preparatory course. That's the only solution I can see.

Thank you for the correction :)