Customer's right denied

I'm an Electrical engineering student.In February I bought a Casio cientific calculator.Its been giving me problems,sometimes it wouldn't switch on,or after switching on it would switch off instantly.It's been rather frustrating since I feel the product is not living up to its standards since I favourite the brand because of its excellent quality.Today I returned it to the place of purchase but to my surprise they refused to exchange it for me despite having had my proof of purchase, I was told to take the same one and I told them I'm not happy about howne has turned out & Im not prepared to take the same one & would rather take a different one even if it cost more I will pay the difference, still I was denied.Im very much disappointed by this & feel my right as a customer is being infringed.How will I use a product knowing well that I don't trust it's performance,.This puts my education at a very risky point because calculation are part of my course & what if it malfunctions right in the middle of my exams.Today I had to borrow my brother's calculator because I was sitting for my final exam & felt I should have back up.I took another of the same kind as per they suggestion