You don't actually need a posh programmable calculator to play games in math class, you can have fun with practically any kind of calculator if you know how to. I have spent hours and hours playing and inventing calculator games, so I think I must share a little bit of my knowledge with you:
Track and Field (those who played the arcade will understand this game's name): A simple game if you don't want to think too much. You need a friend and two calculators. Most standard calculators will count if you press "1", "+", "+" and then "=" repeatedly. The goal is to reach a certain number before your opponent does. If you don't have two identical calculators, you can always play against the clock. A friend of mine is able to count up to 500 in a single minute.
Guess the Random Number: Not very fun, but good for bets. Some calculators have a random number generator. The goal of the game is to give a good guess at the next random number. (for example, guess the last two decimal digits). An easy way to decide who pays the next round.
The Amazing Calculator Game: My favorite, although I guess you have to be a freak in order to actually like this. The idea is very simple: each player must successfully complete "challenges" set by the other player. The general model for a challenge is: given an initial state, reach a given state by pressing a limited number and/or subset of keys. For example, in a Casio fx-82 "Fraction", input "70" and press "shift", and ask your friend to get an error with a single key. (solution: MR/factorial key). There are LOTS of possible challenges like this in $20 Casio calculators (I guess there are even more in expensive calculators) and many of them are really difficult, so it's better to give your opponent three tries.
Some of my best tricks for Casio "Fraction" and "Equation" series calculators (for the "Amazing Calculator" game):
Press the open bracket key eighteen times while your friend can't see you, then input a harmless number (2, for example) and ask your friend to get an error with a single key. Of course, the solution is the open bracket key.
Press the "hyp" key while your friend can't see you, and then ask him/her to do anything with trigonometrical functions.
I hope this will be useful for all the calculator freaks all there... (if they exist)