See, eg, [1] for a chart of teacher salaries vs inflation over time. Quoting the article: "[...] we also find dramatic erosion in relative teacher earnings since 1960." See also figure C, "Real weekly wage trends for teachers and others, 1979 - 2006". In short, teacher's have been taking a hit in the wallet for the privilege of teaching for 30-40 years. In fact, if you're familiar with employment in America, you'll know the quality of teachers has fallen as women have been able to find alternate careers; we used to have a surplus of talented, skilled women who entered teaching because they were unable to pursue alternate careers.
A big part of this is no doubt due to Baumol's cost disease [2], but the trend in wages is real nonetheless.
I'm not sure why people on HN in particular, with an IMO more entrepreneurial mindset than most, are surprised that quality tends to follow wages. In this case, away from education.
When I was in school and played cards on the bus, sometimes we played capitalisim, a fun game if you were on the top of the heap. Invariably the lowest status role was named teacher. Next up was garbageman.