While I don’t believe in doing homework for others, I am rewriting your sentences in the hope that you will understand the differences between the ones you sent me and the ones I returned to you.
To which group do they belong? Teachers must be aware how crucial it is for children to find a group where they are accepted. Furthermore, teachers should be aware of the competition among children.
In trying to rid sentences of dreaded prepositional endings, try being as creative as possible with the other words in the sentence while still retaining the meaning of the original sentence.