The advanced calculators there are these days are really powerful machines.  Some of them have more computing power than the early Apollo spacecraft.  I support all of the Texas Instruments and Casio calculators including the TI-Nspire series, TI-83, TI-84 and the Casio fx-CG20.  The graphics facilities are really useful for sketching functions quickly. The statistics functions are essential for anyone taking the AP, IB and A-Level Stats courses.

Simpler calculators are often used by younger students.  I really advise against using cell phone calculators and the basic calculators used by accountants (usually with large buttons for fast inputting) for schoolwork.  These calculators do not follow the “Order of Operations” rules (called PEMDAS by Americans and BODMAS or BIDMAS by the British).  Your calculator should be a “Scientific” type and you really get to know it well – don’t lose the instruction leaflets!  In my opinion, teachers should spend much more time helping you understand how to use your calculator.