Here’s an idea - stop giving passing grades to failing students. If they haven’t learned enough to get a high school diploma, don’t give them one. Plain and simple.
And yes, adding fractions is elementary, or at least pre-algebra.
First, close the doors of the department of education.
There should be 3 test during a child’s education.
5 grade, 9 grade, and 12 grade. You don’t pass you get held back. Start teaching shop trades again.