Persistent changes in relative prices due to productivity differences I read a couple of years ago that in the long run (in centuries) the relative price between, say, schooling and agriculture keeps increasing, since the latter sector gets more and more productive, while there are barely any productivity gains in schooling.

I couldn't find literature that talks about it.  What is the name of this mechanism, or related literature?
When comparing industries it is almost intuitive that a relative productivity increase causes a relative price decrease in the absence of anything else happening. The reason it is intuitive is that all the competitors within the same industry probably benefit from the same productivity increase so together you can all raise prices slowly or not at all or decrease prices. 

You might be looking for the Baumol effect. Wikipedia says...


  The rise of wages in jobs without productivity gains is from the
  requirement to compete for employees with jobs that have experienced
  gains and so can naturally pay higher salaries, just as classical
  economics predicts. For instance, if the retail sector pays its
  managers 19th-century-style salaries, the managers may decide to quit
  to get a job at an automobile factory, where salaries are higher
  because of high labor productivity. Thus, managers' salaries are
  increased not by labor productivity increases in the retail sector but
  by productivity and corresponding wage increases in other industries.


According to this, a necessary condition for the Baumol effect is competition between industries for resources. 

This is the relevant link.