Every stage of school has been designed to challenge and push the skill levels of the students who enter into it. After all, if you are not going into a new level of school facing some challenges, then there is either going to be a wide gap when you finally get a level of schooling which does challenge you (such as the transfer from high school to college) or you are not at the right level of schooling for where your skills actually sit (which really has very little to do with chronological age at all). Grammar school is just one part of the process, but being early and designed to build skills, it could easily be argued to be the most important step of all.
First off, grammar school’s first challenge is in getting students (who have previously been subject only to the whims oftheir parents in regard to their schedules) into the rhythm of going by clock time, and dealing with boredom. School in its current form was originally developed to condition people to the jobs they were expected to get (doing things such as office work or factory work), and these little discussed skills are essential. While it is the foundation for later schooling to be able to handle these elements, they are just the beginning.
In grammar school you are also just learning the basics of the academic skills which will help your students to power through theirr future educational endeavors. Reading, writing and the ability to work with numbers, as well as the basics of critical thinking should be stressed at this level. If they are not, then the students are going to face a very steep climb in high school, and may ultimately end up failing miserably. After all, one can only learn so many different things at a time. Learn the most important things as soon as possible.