I agree about introducing that system in Japanese school. If children are always conscious that policemen are patrolling a school, the number of the problems of bullying will decrease. In addition, this system will prevent teachers from hitting their students or use violent language to them, so parents can trust teachers more. And when a suspicious person breaks into a school, policemen can protect students. soa natural?

(I'm assuming this is the introduction to the essay. First paragraph?)

The passage is completely natural, if we look at it from that perspective. If I were to look at it without knowing it's written by a Japanese person, I'd assume it was by a native speaker. In fact, most of my classmates could probably not write as well as this.

That said, I'd edit it to make it "stronger", not more "native-sounding". If I were to mark this passage, I'd probably give it a level 3 (in Canada we have levels 1, 2, 3, and 4, 4 being the highest), so around a 70-75%? Personally, this is how I would edit it:

"The school police system should definitely be implemented. If children are aware that policemen are patrolling the school, the number of the problems related to bullying will decrease. In addition, this system will prevent teachers from hitting their students or using hurtful/abusive language (language can't be violent), so parents can trust teachers more. And when a suspicious person breaks into a school, the police will be there to protect students."

However, if I were to write the essay from scratch, I'd put it slightly differently. Note that I don't know the state of school security in Japan, and I'm making guessing based on what you wrote.

Here's a level 4+ first paragraph:

"Recently, security issues at school have begun to increase at an alarming rate, especially compared to the past. Is it so unreasonable for parents to expect their children to be in a safe environment when they send them to school? In the past, schools served the purpose of being secure institutions to nurture and educate the people who would grow up to build Japan's future. In today's world, however, it has become more the opposite. Schools have become breeding grounds for bullies. Hundreds of children are driven to self-harm every year. Parents are left completely dumbfounded when, in extreme cases, the children end up committing suicide, because, due to constant bullying and insults, they learned to suppress their insecurities, opening up to no one. Even some teachers abuse their students, physically and verbally. Where does that leave hope for the future? Implementing the school police system would solve that problem. If the students are aware that the school grounds are being patrolled by cops, they would be doubly mindful of themselves, and several times more cautious while bullying another student. Teachers, also, would not be able to abuse students, knowing it would most certainly lead to arrest. Even if a suspicious person were to break in, the school police system would allow cops to ensure the students' safety."

Whew, that was long. Sorry about that. But yeah, your passage was natural, but the edits I made will probably be stronger. Forgive me if I made any typos; that was way too long to proofread.