Adding this element to your ensemble requires a few different components. The keyboard itself, a stand, pedals, cables and some kind of amplifier in order to be completely self-contained.
Let's start with the keyboard itself. You have to make a couple of choices here – go with something that really approximates the sound/feel of an acoustic piano, or something that can be both a piano and a whole lot more. A nice choice in an 88-key weighted-action 'stage piano' is one of the Yamaha models. These are really durable with a nice heavy-duty casing. Known for great sounding acoustic pianos, it's no surprise that the Yamaha piano sounds are excellent along with a natural-feeling keyboard. Roland also makes a nice quality product with a similar level of portability as other brands. The benefits of a great feeling and natural sounding digital piano is a tradeoff vs. additional sounds and flexibility that other keyboards will feature.
A slightly different option might be to go in the direction of a 'workstation' style keyboard. Here you'll have a few options for either weighted or unweighted keys, and choice for 61, 76 or 88-keys. Personally I like the idea of a 76-note (or even 61-note) unweighted keyboard to use for this jazz band type of application. The 76-note range allows the player to hit the key low notes when needed, and the un-weighted keys make the unit a little more portable (lighter) vs. a weighted 88-note keyboard. The Yamaha keyboards in this style again have great sounds (thousands of them!) along with being flexible for any kind of musical situation. Korg is also a well-known brand among keyboard players and features several models that would work nicely in a school setting. For the really adventurous educator looking to take their jazz band to an entirely different level, check out the options available from Hammond (yes, the same Hammond as the classic B3 organs). If budget wasn't a concern, I'd love to see a jazz band with the option for a traditional HAMMOND ORGAN sound on blues or rock chart. And it's a great opportunity to teach the kids about the classic sound that is really at the soul of the Hammond organs.
Once you've decided on your keyboard, a portable stand is needed since you'll need to use the keyboard in (hopefully) many different locations. A simple "X-style" stand is really the answer here and the ProLine PL400 is the perfect choice. Combining double-bracing for extra support even for heavy keyboards with lightweight portability is the key (pardon the pun). The X-style stands can accommodate either sitting or standing. And for those of you who are adding both a larger digital piano with a smaller Hammond keyboard (you know that's what you want to do), the ProLine two-tier stand will work perfectly.
The choice for a quality keyboard amplifier is pretty simple, go with a Roland. With a number of different sizes to choose from it's easy to think about saving some budget here. But try to stick with something that has at least a 15" speaker which is really needed to reproduce the low-end of your keyboard. Specifically the Roland KC-550 is used by thousands of schools every day. The power, flexibility and the fact it has wheels makes this a no-brainer for your jazz band setup. Of course, if you want to capture the Hammond sound, you can even use a classic Leslie style amp. What would be cooler than rolling in a Leslie speaker into your jazz festival!
To round out your keyboard setup you'll want a damper/sustain pedal and perhaps even a volume pedal. The volume pedal while being optional can also be quite useful to capture a swelling string sound, or simulate the 'pop' of an organ voice. Finally don't forget about a case for your new keyboard. If you have something with 76+ keys, consider a case with wheels so your students don't have to team up in order to carry the keyboard.
With these options you'll have both a practical and a portable keyboard setup that is flexible enough to either take the place of your acoustic piano, or even explore some new sounds with your rhythm section. Good luck!