A graphing calculator is not required for any math course, but many students find that it enhances their learning, especially in Algebra II and beyond. It's important to consider the price of these devices, as they often cost upwards of $100. They are indeed an investment, but they are useful in a variety of math and science courses extending well beyond the scope of a single school year (and we all know how rare it is to find a school supply that lasts beyond spring break!). Below are some recommended calculators, listed in order of popularity in math classes over the years (from most popular to least).
TI-84 PLUS: This calculator appears to be quite popular and easiest to use, according to last year's Algebra II students. It's not the cheapest one available (over $100), but it performs a number of functions that help students visualize Algebra II concepts.
TI-83 PLUS: While usually a bit cheaper than the TI-84, it performs almost exactly the same functions. Its interface is a bit older, but it does all of the same things you'd need it to do in Algebra II. Used TI-83s can often be found used on sites like Amazon for very reasonable prices.
TI-89: This calculator is PACKED with functions, most of which are useful in Calculus and beyond. It's more expensive (closer to $150), so many students prefer a less expensive calculator until they reach a math course in which a TI-89 can actually be used to its full potential. One major point to consider: since the TI-89 has a built-in algebra system (which means it can solve equations), it's banned on some tests like the ACT and in some college courses.
OTHER TI CALCULATORS: Texas Instruments (TI) makes several other models, some older (TI-80, 81, 82, 85, 86) and some newer (TI-Nspire, TI-92). All perform similar functions to the calculators listed above, with some variation in price. La Serna's math teachers are predominantly familiar with the 83 and 84 models, so students often report that those calculators are easiest to use in classroom settings. We teachers can usually figure out the other models, too (albeit with a little Googling in some cases), but it may require extra time, some trial-and-error, and some patience!
CASIO: This brand is often much cheaper than its TI counterpart. Casio calculators are often comparable to TI's in terms of functionality. However, most of La Serna's teachers use TI calculators in class and are more familiar with the TI interface. Casio's interface and button layout is much different, so students who buy a Casio calculator should bring their user manual to class daily in case support is needed.
HP: The HP brand of calculator is comparable to TI in terms of price and function. However, very few of my students seem to buy this calculator and few of our teachers are familiar with it, so proceed with caution -- as with the Casio brand, it will not be as easy to follow along and receive support during classtime if calculator issues arise!
PHONE-BASED APPS: Students with smartphones may be inclined to purchase apps simulating a graphing calculator at a fraction of the price of the device itself. A particularly good free app is called Desmos. While this is certainly a valuable asset at home, it can be tricky to rely on a phone-based app in class -- especially because many of our teachers do not allow the use of cell phones during classtime (believe it or not, a few naughty youngsters have claimed they were "using their calculator" when really they were texting their friends -- I know, I know, YOU would never do such a thing...). Also, phones are prohibited in many testing situations such as AP, SAT, ACT, and college entrance tests where other calculators are allowed, so many teachers advise against relying solely on apps.