I teach AP Calculus and Geometry at the American Embassy School in New Delhi India which had a slightly different approach to the same problem.
When we arrived my son was entering 6th grade and we were told unless your son has problems in math he should go to regular grade 6 math which was the seventh grade book having skipped the 6th book(this fact was known at the time).
The motivation was to have virtually every student take Algebra I in 8th grade thereby enabling them to take AP Calc or IB HL.
Needless to say, he and others had trouble in the new topics from 6th which weren't introduced but were expected to be known. My son was one of the luckier ones since he had a built in tutor. There has been a snowballing effect each year as mid to low students have become less and less prepared each year. We are now trying to design courses for these students at the high school which will recapture the enthusiam for math and still prepare them for college.
The whole program was completely revised this year to include a standard 6th grade and bridge to algebra 6th grade. Students then take either a one or two year pre-algebra and then the standard algebra I course. There are also many crossover models built in to enable students to move back and forth so they aren't bound for the rest of their career in math by their 5th grade teacher(although we have great 5th teachers, my wife being one).
I am hopeful these changes will generate students with better attitudes and skills in mathematics while allowing students the chance to take AP Calc if they desire.