My younger brother is in his second year of junior high school this year, and it is good for him to

According to what you said, if your brother does not deliberately mess with your classmates, the other party's parents have no right to find you. And because both of you have not paid insurance, even if the school is not very responsible, but for the sake of your classmates and friends, you can contribute some money to show your personal affection. You also have to give feedback to the head teacher or principal of the school, if the school really ignores it, and it is somewhat necessary, then the parents of the students who do not pay the insurance should pay the main expenses themselves.

The school should be responsible for accidents at school. But your brother and that person should also be half of the blame.

The school shall bear the responsibility for accidents that occur to students at school.

As for your brother's classmates, it is completely unreasonable for the family to hold your brother responsible.

Is it the relationship between the ground and the slippery ground? If so, that's the school's responsibility. You can ask the school for compensation.