stracehas proven useful to me in the past (though I'm not certain how much the students got out of it): it shows which system calls even a small program will make.
In the course of trying out some new exercises around
strace, I discovered that the man pages for system calls aren't installed by default in Ubuntu. Loading them is just an
sudo apt-get install manpages-dev
I hope my students don't get a nosebleed. Nor me, either.