Students who fail to learn to read in the primary grades rarely develop into great readers in middle and high school. They generally enter kindergarten behind, read two to three years below grade level in elementary school, and are still two to three years behind their average classmates in middle and high school. Districts that lack the organizational will to teach their students to read at or above grade level by second or third grade, when it is relatively easy and inexpensive to do so, rarely get them to grade level thereafter when it is much harder and more expensive.
That doesn't leave much wiggle room. Get to work in primary grades when you've got a fighting chance to make a difference.