ardly:
- To separate items in a list.
- To provide pause for breath and perhaps emphasis.
- To bracket optional additional information that can be omitted.
All three examples are pauses of lesser or greater extent.
"In the above there is no list, the commas are not for breath and the sentence "In school but some students do not realise that" does not make sense."
I beg to differ(!)
"In school diligence is desirable but some students do not realise that. (sic.)"
'School' and 'diligence' are both nouns
Two nouns may be conjoined with a hyphen...
"In school-diligence is desirable (!)"
...Or they may be separated, as a list.
"In school, diligence is desirable but some students do not realise it."
I am happy with a single comma or even better none.
Technical English is written using open form punctuation, omitting all that is not essential. English language students are taught the more expressive and pedantic, closed form, punctuation.