GolamMostafa:
In punctuation test class, my non-native tutor would expect that I paraphrase the above quoted sentence as:In school, diligence is desirable; but, some students do not realise that.
I don't like the comma after school because it suggests a disconnect between school and diligence and it would certainly not be appropriate if the sentence was better formed as in "diligence is desirable in school"
I have no problem with the "but" but I would not put a comma after it, and maybe not before it either. There is a sense in which the word "but" acts as a comma.
And I think you mean "rephrase" rather than "paraphrase" - or, even better still, "punctuate".
I remember a quotation "I am writing you a long letter because I don't have time to write a short one" (though I cannot remember the source). In my experience most writing can be usefully shortened (and made clearer) if the author takes some time to do so. (Including mine, no doubt)
Legal documents often have no punctuation other than full-stops in case the punctuation causes confusion. The "Oxford comma" is an example.
... R