Well now you know if you go missing your guardians are going to have a problem with it. As if you didn't already know.

It is a crime even if your boyfriend is a minor. Even if he isn't liable, then his parents are. If your grandparents are your guardians, don't be surprised if there's an issue with you missing overnight. You aren't emancipated yet. Deal with it. Everyone does.

Your grandparents are liable for your well being. Your mother's permission doesn't mean anything and you know it. Technically your boyfriend committed a crime and contributed to the delinquency of a minor. You've gotten off fairly unscathed, be more mature next time.