For those who feel that your kids who are not receiving as much work as the kids in the top classes. Try speaking to a few parents to see if it is true in their case too, Please do not be surprised that this is a practice is shared by many primary schools in Singapore. So, it is not only in a particular school that the top 2 classes get the "most attention"
I have a friend who used to teach in a rather popular school. She has since retired. She offers a different perspective. She explained that usually the top 2 or 3 classes in the school are deemed as "students who are better positioned to undergo stress-testing" ie like u mentioned, where the top 2 classes are given more practice papers. My friend said she has not seen the practice papers of SSS so she will not comment but from what she understood , these practice papers do sometimes "add stress" as certain questions appear to be a little more complex which is why it is often given to the top classes where the students can handle the paper and yet not feel "atrociously pressurized by the sheer volume of additional work nor the complexities of such papers".
Give the same to the "supposedly academically less-inclined" students, and they may feel daunted by the task and it is not healthy to "dampen their esteem/ morale" in these crucial years. What is important for the "non-top 2 classes" is revision & revision to ensure consistency after consistency so that they walk into the exam hall feeling prepared and with confidence. As the saying goes, confidence is half the battle won. So, simply giving them more practice papers than they can cope may work against them, dampening their confidence-level/ morale.
This further explains why classes are often grouped according to the abilities of the students so that the group learn at their "supposedly similar pace" . Put a child who is not as bright in a GEP class, I am sure the child will suffer not only academically but may even bring about detrimental effects on the child’s overall developmental milestones in life.
Speaking to my sis with a primary school going kid, she shared that her child’s school also has this unspoken practice of giving the top classes "more work" or as you had described "more attention" as the school probably feels that this group of students can cope and will not feel adversely affected by the additional workload.