A couple of suggestions to make things harder. A lot of these assume that you're doing this somewhere at school, not that you're sending the CDs home (if you are, give up: the student could just cheat using another computer, phone, tablet, etc.)
Instead of having the entire OS on the CD, only put a minimal system on it. Preferably only through initramfs. Have it use an NFS root filesystem, on a server you/the school controls, and can only be accessed on exam day.
This way, the test taker has a much harder time examining the system; he/she must do it during the exam. The test taker of course should not have root access, and no persistent, writable media should be mounted, so he/she will have a hard time
tar'ing up the rootfs to examine later.
Also much easier for you to update. You don't have to burn 200 CDs each time you want to.
If all the machines support netbooting, netboot them. Do not distribute CDs at all.
Make it look different
Linux desktop environments are very customizable. You can (depending on which one) change the color of the windows, title bars, the fonts used, the look of the menu & task bars, the desktop background, etc. Don't do anything crazy (that'll make the system too unfamiliar), but do make it non-default. Then you'll be able to quickly glance at screens and see the one that doesn't look right.
You could also do smaller things. E.g., put some icon on the desktop, or an analog clock in the menu bar. When you look closely at a screen, you can check for these.
… each exam
Someone might be able to copy your look and have it ready for the next exam. Or at least close enough that you have to look carefully.
If you don't have to re-burn images all the time, its easy to change the appearance each exam. The student won't know what ExamOS will look like today until he/she starts the exam.
Set up a VNC server
You can set VNC up to allow you to view the desktop, so you can check on people from the comfort of your desk. Ethically, I think your should notify people that you can do this.
If you have the fileserver space & bandwidth, do a screen recording of the entire exam session. (There are several screen recording programs). Review only if suspicion arises and/or a random sample. Another one which, ethically, I think you should notify students of.
Control the media
You should give out the ExamOS CDs at the beginning of the exam, and collect them at the end. Require that people turn them back in.
You can block all outgoing traffic except the traffic you'd like using iptables. I'd suggest you configure the web browser to use an HTTP proxy, and the only outgoing traffic you allow is to that proxy. Additionally allow DNS if needed to connect to the proxy (but only to your DNS servers).
You can configure squid, apache, etc. with a list of URLs that are allowed and disallowed. You can run this on a server you control, or each local machine.
If all the exam machines are behind a firewall you control, block things there, too. The student won't be able to bypass that, without bringing their own connection.