A wireless sensor network for a project school seems to have a great output when it was successfully done. Wireless is more than applicable nowadays in all conditions. This school project is quite interesting and yet can be used to the vehicles.
I'm a PhD student that has worked with WSNs for the past two years, so I don't think I'm out of line when I say I can claim some authority on the subject.
For hardware, you could have a look at redbee (http://redwirellc.com/store/index.php?route=product/category&path=37), which is an ARM7-based MCU with integrated transceiver and a bunch of I/O. I believe there's a Contiki port for it so you can easily set up a simple network (or even IPV6 if you prefer).
If you're looking for something really cheap, I think Dresden Elektronik still has a couple of these: http://www.dresden-elektronik.de/shop/prod28.html. It's an Atmega1281 combined with an RF230 packet radio. It's cheap apparently because they since produced a new version, and you'll need to buy one or two breakout boards with it, but it's an extremely good deal price-wise.
These are just two suggestions, feel free to browse around.
On the software side, you might want to think about what you want to accomplish first. From your description you just want to do some sensor readings and send them to a central point (sink). There are no radios I know of that cannot get a data packet from one side of a car to the other, so you're talking single-hop, meaning that you assume every transmitter can get data to the sink directly without going through an intermediate party.
What you need to accomplish this is a radio chip, a MAC layer, and a network layer. The MAC layer is a piece of software that runs on each sensor node, and makes sure that two nodes don't talk at the same time. Almost all modern transceiver chips are packet radios, making the MAC layer much simpler than it used to be. The network layer is something you most commonly need when you're doing multi-hop communication, in large or sparse networks. In your case, you don't really need it because you can just wrap each measurement in a single data packet and ship it off to the sink.
This is an arduino forum so naturally people will tell you to get an arduino+xbee sandwich, but this is expensive and overkill. There are literally loads boards out there that are designed specifically for sensor networks, you could try one of them. Those XBEE modules have their own processor on them, which handles the MAC and network layers, while your Arduino just reads the data from the sensors and sends it to the XBEE. This is redundant because now you're paying for two chips, a bunch of PCB board to connect them together, and a load of other superfluous parts. You can integrate the whole thing into a single board if you use a good middleware solution. You can choose between TinyOS, Contiki, or roll your own.
Don't use TinyOS. It is very difficult to work with and the tools suck ass. It's use is mostly academic. For a piece of middleware that works out of the box try Contiki (http://www.sics.se/contiki/), and see if you can get some hardware to match. You can also roll your own stack, but make sure you have some C experience before you try something like that.
Finally, my advice would be to just run some wires. Seriously guys, it's a car, it's supposed to have wires in it.