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You could make the exact same argument about science though - all of science is just a series of mathematical models that are able to predict with some certainty what happens in the universe we live in. However, there are no exact models of anything more complicated than a single hydrogen atom in an otherwise empty universe - anything remotely approaching the complexity of the physics, chemistry, biology etc. we experience on Earth are largely empirical models (i.e. measurements of the effects of variable x on y), and once you get into broader sciences such as environmental science, psychology and sociology all bets are off.
Because of the similarities, it is often debated whether economics, or at least parts of it, is itself a science - there's a fairly good article on the subject here: https://www.theguardian.com/business/economics-blog/2013/nov/06/is-economics-a-science-robert-shiller
My point in all this, is that teaching Economics doesn't fundamentally have to be less rigorous than the teaching of sciences, for example. Also, as we're talking about secondary school here, the mathematical models would of course need to be kept to the very minimum with most of the emphasis placed on giving an overview of the qualitative aspects of the subject, as well as practical information on managing personal finances.