Losing proposition?

I'm going to approach this topic from a slightly different angle bases upon the conversations I've had with my sister who is an educator of elementary school students in eastern Montana.  She has approximately 20 students in any given year in her classes.  Our discussions have often focused on the students that are falling behind in class or were behind when they started her class.  Those students tend to have learning disabilities of some sort, we rarely discussed specific disabilities, but instead discussed how she was to approach educating the entire class to prepare them for the next year of schooling and beyond.  If my sister felt inclined she could spend the entire school year focusing on those few students and gently bringing them along until they reached the necessary milestones, proficiency reading, arithmetic, writing, etc.  However if she spent that much time ensuring the students with disabilities were keeping up then the average students and the above average students would quickly become bored and not reach their full potential in the year.  How do educators find the balance between keeping their average and above average students focused and moving forward without leaving behind students with disabilities?  As my sister has found most teachers prior to her just focused on the average students and essentially just passed the below average students onto the next year without caring whether or not they were prepared.  

This brings me to a point where I can see both sides of the argument.  The majority of the students were prepared and educated to a sufficient point that they could move along in their academic education.  The small minority that wasn't able to keep up was ignored or given an insufficient amount of attention to succeed.  So where is that balance?  Do we insist that our educators provide more attention to students that can do less with it?  That is a harsh statement but the upper level students are more likely to go onto college and contribute more to society whereas the bottom level students will likely provide less to society after completing, if they do complete, school.  But, are the lower level students contributing less because no one could or would spend the time necessary to bring them along?  Or were they going to end up their regardless of the attention given to them?  I certainly don't have the answer to that but it begs for debate.  Do we spend more time with students that struggle and the odds say are not going to make it through college anyway, or do we focus on the students that are likely to succeed?