We say school of economics. The most famous one in the world being: The London School of Economics, or LSE for short.
economic school is an error.
- school of law (also law school), school of economics, school of design, etc. But very often capitals are used to refer to an actual school:
Rhode Island School of Design
In discussing educational institutions, school of economics is the right term. Sometimes, you can use a noun as an adjective: law school. It depends on the discipline and the word. It can get rather complicated. A school of philosophy is the same as a philosophical school, but in this case does not refer to a physical place. However, a cooking school would be a physical place.