Could you look at the following passage and point out any mistakes?

"Schools in the US have the grade system, which is to separate students into groups by their chronologic age. That system has several drawbacks. It doesn't take into account child's readiness to begin studying. While some children are ready to go to school after they became four-five years old, some other children are not enough mentally or even physically mature at the age of six. The grade system doesn't take into account that different students have different development rates. Some students, who need more time for studying, have no time and those, who need less time for studying, have to wait for others. Another doubtful point is that students, who haven't learnt enough and donít meet requirements for the next grade, have to repeat the year. As many researches affirmed, it doesn't help much, but causes loss of self-concept. Overall, the grade system evaluates students in a group instead of using an individual approach, which makes that system at least not ideal."

Thanks in advance.