Why in the mathemathic classes everythig must be perfect?

Because one small error can give you an entirely different answer than the correct one. for example;

10 = 2x + 2

let's say instead subtracting on both sides first you divide 2 on both sides and get this; 6 = x, which is the incorrect answer. The correct answer would've been 4 = x if you had subtracted first. You got in incorrect because you mixed up one step of your order of operations. So you see, every step you do in math, even the order is critical in finding the correct solution.