Allowing the graduates to continue on, in what I consider the most important time is essential.
The issue is when they leave the kindergarten /daycare they are proficient in English. But primary school with all its education will dampen and smother that English development very quickly. 
Plus the influence of Katakana the accent changes, and in 3rd grade when students start learning Romaji...😱

At the school we have to be vigilant for the accent degrading and output decreasing.
It's a difficult period also because by 3rd grade if not sooner the students want to start doing other extracurricular activities, clubs etc which also leads to a decrease in English output time.
Saying that by 4th grade the brain has pretty much locked English as a 2nd language in.