Oh geeze! I'll try to find a paper on this, but it varies a lot in the US's K-12 at least, hence the 'rule of thumb'. The source I have off the top of my head

Oh geeze! I'll try to find a paper on this, but it varies a lot in the US's K-12 at least, hence the 'rule of thumb'. The source I have off the top of my head is family members that are in education consulting and therefore talks over dinners and whatnot, but that is not a 'source' in the HN sense.