Coming from a high school sophomore who has been detained by the police only once for having fun (wheelchair racing through the local supermarket), take this with a grain of salt.

What they did was wrong. They phoned in sick and concealed their identities. They also disrupted the classroom and school environment, similar to a fight (conveniently called a "campus disruption" in politically correct terms) or a pulled fire alarm.

Now, senior pranks will exist until the end of time. I plan on doing one, but am honestly deterred by the punishment on not walking at graduation or going to prom. I don't want a basically squeaky clean record tainted by one day of fun. Maybe that'll change.