Here is a quote from Sally Shaywitz's "Overcoming Dyslexia, A New and Complete Science-Based Program for Reading Problems at Any Level" p. 261:
"In one Tallahassee, Florida, elementary school where such a program [scientifically proven prevention and early intervention programs] was implemented, the percentage of struggling readers dropped eightfold--from 31.8 percent to 3.7 percent."
I would guess 1% using Webster's Speller and teaching both spelling and phonics at the same time, its focus on spelling and syllables is very helpful for my struggling students.
BTW, that's a great motto, "Education not remediation." Every smart students learn more when taught explicitly.
The Shaywitz example that Elizabeth quotes is a fairly representative one. Schools that use effective instructional methods and materials significantly reduce their "struggling reader" populations, usually to a 1-digit %ile. There are many examples; most however are school-based. We don't see scaled-up replications of this type of success on a district basis.
The 1% that Elizabeth cites is reasonably the percent of children who would still flounder even after intensive 1:1 teaching with effective methods and materials over a significant period of time. However, it would not be likely to represent what could be achieved in general education classrooms at our current level of knowledge and operational constraints.
Students with severe disabilities must be taken on a case-by-case basis, as their learning needs are highly specific; in other cases, important variables are not always under school control -- for instance, students with high levels of absenteeism and/or frequent changes of residence. You can have the best-ever teaching, but the student has to actually be there and participate in order to benefit.
So hard data on what is the lowest percentage of students who we can expect will still lag *far* behind despite our best efforts is not available. What we do know is that the number of "struggling" students is far too high.
From intensive work with students at all ability levels, I believe it's a testable hypothesis that 99% could be taught foundation skills in reading, writing and math that are commensurate with their level of receptive language comprehension and cognitive functioning. Some cases would be extremely labor-intensive, and this becomes a time and resources issue. Public schools realistically do not have enough instructional time to be as successful with some high-needs cases as private services can be. I can think of cases where children have needed 4-6 hours a day of intensive therapy in several areas to make significant progress. Schools are not set up to provide this, and perhaps it's a separate question: should allowances be provided to parents of such children to purchase the needed services using special education funding? Should the school district contract these out?
We are talking about a small number (percentage-wise) of students, but their needs are real and not easily met.
Getting all students reading at the K-2 level is realistic for most schools. It is, however, only a step in the process. Children who master decoding and spelling early on may be strugglers later. That is why careful progress monitoring of all students is needed.