If you need to display a working scientific calculator to the whole class, look no further.

http://www.ecalc.com/calculator/scientific?ca=BCALC&typ=MIO&src=USB&vrs=...

You can also download a free virtual four-function calculator that can be carried with you, eg on a memory stick.

http://www.ecalc.com/files/mio/ecalc/basic/usb/calculator.exe