"I am writing you a long letter because I don't have time to write a short one"
Consider then, perhaps, a hyphen; "In-school diligence is desirable".
I am writing you a long letter; because, I don't have time to write a short one.
Re: Subject line.Etcetera means "and other stuff" so you don't need the "and" before the "etc."
But almost nobody says "and etc."
I had not given it a lot of thought but I assumed the title meant that the OP wished to discuss the use of "Etc" and the use of punctuation.In other words I subconsciously interpreted the title as "Etc" and Punctuation Marks of the English Language
I have two items in the Title, which are Punctuation Marks and Etc.. What else is there except the coordinating conjunction and that can help me to glue these two items?
I read it as; In school, diligence is desirable, but some students do not realise that.
In my limited understanding of grammer
To separate items in a list.To provide pause for breath and perhaps emphasis.To bracket optional additional information that can be omitted.
"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 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.