I do not think that the evidence goes much beyond the romantic and nostalgic view of what school used to be. The visions of bringing a red apple to Mrs. AlwaysgivesanA and kids walking or riding a bus to school is the image. That an the sandlot summers in the bucolic suburbia Shangri-Las with cars that one only see in La Havana nowadays.
Perhaps we could assume that the schools were good. The rich and connected surely got an education that appeared more rigorous in the early 20th century, but the reality of a segregated community before the MLK days surely challenge the idea that schools used to be good.
Setting all that aside, the real issue is that the US, despite spending more than about every nation of earth on a per capita basis (nor a GDP percentage) has not stopped the descent in relative mediocrity. We are going the opposite direction from the rest of the industrialized world. If we maintain a lead in tertiary education, that is often a proxy for having very rich private universities and colleges and a dedication to funding research with public and private funds.
And the biggest issue remains the long term survival of the current model that marries a free K-12 (or purportedly free) with a tertiary education that saddles families with ever growing debt. In both cases, the education world is one that has been spending freely and without much restraining forces based on the belief we can always borrow ourselves out of reality.
It does not matter if the education used to be better; what matters is how we climb out of this planned mediocrity where the lesser talented is tasked to educate the next generations and where doing the least for the biggest rewards is the motto.