All throughout my schooling, from year 7, 8, 9, 10, 11, 12 as well as my first 2 years of university, i have been using an old Casio fx-82TL Calculator. Although only now as i've been getting into property economics i've needed a new calculator for complex equations, this calculator has been the best calculator i've ever owned.
It's a scientific calculator with all the basic scientific functions, minus what modern calculators have with they're graphical charts and extra formulas. At a price of $49.99 eight or nine years ago, the casio has withstood thousands of drops, punches, kicks, rain, basically everything you can throw at a calculator, without any damage what so ever.
At 7cm by 16cm, although outdated, keeps up with the size of modern calculators, without the problems of reliability. I say this, as a new calculator i bought only a week or so ago, has had issues with the battery. From what i can remember, this calculator has only had one battery change in it's life, meaning the chances are of it going dead in an exam are highly unlikely.
The calculator comes with a cover that can be attatched under the calculator, aswell as all the basic calculations can be kept in a small pouch in the cover.
So next time when your considering a calculator for yourself or a child for school, think about how long they will need it, and consider a casio calculator, which I believe are a trustworthy brand in calculators.