Many high schools recommend or even mandate it: the Texas Instruments TI-84 Plus CE-T graphing calculator. This model (TI-84 Plus CE-T Python) contains all standard functions and the new Python app, which students can use to get acquainted with programming. This app is disabled by the safe exam mode and thus makes no difference during the exams.
The modern calculator has a brilliant colour display (high resolution). Sums, graphs and formulas are very clearly visible on this. Thanks to the included USB cable, charging is just as easy as with a smartphone; familiar territory for most students! It's also no problem to take it with you in the school bag, thanks to a thin design and sturdy, shock-resistant lid.