I completely aced the national math exam to enter university. Bringing a vintage HP-15C was one of the key reasons. They forbade alphanumeric and graphing calculators to avoid students storing text or plotting functions.

But they didn't know that non-alphanumeric / graphing calculators can still implement sophisticated numerical methods. And in case the calculator doesn't have whatever you need, it can always be programmed.

I used these algorithms to verify huge linear algebra and calculus problem solutions. Plus, learning RPN makes calculating a pleasure and can motivate the student to understand other CS concepts. The 15C or the 42S are great choices.