1) There are almost no grammatical mistakes.
2) The changes are for style: a few clumsy places, no more.
One hot and humid afternoon, school had just ended. Jack, James and John saw an injured stray cat. Instead of pitying *it, *they, being naughty, threw empty cans at it. It went behind a garbage bin, to avoid being hit by the cans.
* it is correct. But to give the cat more character, would you like to consider using she or he as the pronoun?
* But instead of pitying it, naughty boys that they were, they threw cans at it.
“Ha! Ha! Look at the frightened cat!" James said. The other boys were amused. They continued throwing empty cans at it. “Meow! Meow!” *the cat mewed as blood *was oozing out of its head.*
*I don't usually suggest verb-subject inversions, but here it is good to keep the three mews together. mewed the cat
*strong! suggest oozed out. I know the dropping of the continuous tense is hard to explain in terms of pure grammar. Here its omission makes the style more solemn: quite fitting for as awful a scene as you have imagined.
“Hey! We should use our water guns and spray water at it instead,” John suggested. The other boys agreed and soon they were happily spraying water at the helpless cat. It mewed and mewed while the boys continued to torture it.
“Stupid cowardly cat!” John laughed as he criticised the cat. The other boys laughed too *as they found John’s remark amusing.
*suggest laughed too, so amused by John's remark. This is not a matter of grammar, just of weighing each word.
“Why are all of you torturing a helpless cat?” a voice shouted. The boys turned round and were shocked to see their school principal. “I’m going to call your parents.” They asked the principal to give them a chance, but she did not budge.
She asked Jack to take the cat to the school and later asked one of the teachers to take it to a veterinarian for treatment. She informed the children’s parents about their cruel deed. Their parents were so angry with them that they promised the principal that they would punish them when their children returned home. The principal thanked them for their cooperation in disciplining their sons.