Eh I think that is somewhat true but there are certainly middle class kids who learn how to fight. I think it's more that Leinster systematically plan to mimimize the amount of adversity they face on the rugby field through out the year. Playing in the Leinster school cup is hard, but it's not the same as playing 30 games a year in the top 14. Playing 4 games a month may not be the best thing for your body but it builds character.
I was careful to note its not a class thing. Its a school of life thing.
Not sure you can say the 4 games a month is it when Sexton is probably the attitude leader in Leinster and POM the same in Munster.