I think you'll have to do that research for yourself, but I did a cursory googling for you and it seems fairly stable in the US since the 70's [0]. [0] https:/

I think you'll have to do that research for yourself, but I did a cursory googling for you and it seems fairly stable in the US since the 70's [0]. [0] https://nces.ed.gov/fastfacts/display.asp?id=38 https://nces.ed.gov/fastfacts/display.asp?id=38