Severity

He's 17 years old, he doesn't even have a fully formed brain yet much less the rational decision making to evaluate the long-term consequences of something like this.
A whole lot of 17 year olds have done a whole lot stupider things than than this with a lot worse results for everyone involved.
As far as harmless, victimless crimes go it's pretty benign. Choosing to drive home from a party drunk is a worse choice than this for a 17 year old kid and it happens a thousand times a year.