The casper-files and their vulnerability?

[Are Venes]

  Hi

I am making a bootable live-system out of my Ubuntu 10.04.
The thing i am wondering about is the socalled casper-files.

I have done some changes to /etc/fstab and /etc/network/interfaces after
making the bootable system(on the running system).

The question is, will my changes be reverted if someone inserted "blank" 
casper-files
into my system? Really thankful if anyone can explain how this is done. 
What exactly does
these casper-files contain, (details probably not needed, just 
explanation to help me understand)

Got some help earlier with making some of the casper-bottom scripts not 
run at boot time.
How are the casper-files merged with the rest? Is it only scripts, or is 
it more to it than that?

The system will maybe be used in school-situations(by students), so 
security should be somewhat as good as it gets.

The worrying bit is: Someone steals the system, boots it from another 
linux, replaces the casper-files
and gains access to resources that he shouldn`t have during a "exam", is 
this possible?

Regards Are