cont:  They will fall further and further behind as they move through succeeding years.  Schools often provide remedial "tutoring" to help.  But, receiving such tutoring precludes being in the "regular" class as it moves forward, and a student seldom really "catches" up.  Summer sessions may be helpful to avoid this issue.  

However, regular class teachers are "forced" to gear their teaching to the needs of the students in the class.  Realistically (and problematically) this usually means teaching at or near the "lowest common denominator" of the class.  Ability grouping, which is opposed by many as "discriminatory", can address this issue.  However, this means a portion of students will move "faster and further" than those in a "lower" ability group.  

IMO, we need to move beyond the idea that each year kids will "advance together" and, instead, have students move forward based on their development, not their chronological age.  ..would enjoy visiting w/you on this one   Gary