Parenting Rulebreaker

I recently broke a cardinal rule I always preach to my teenagers about privacy. One night, after they went to bed and thinking it was for their own good, I peeked at their social media messages. What I found shocked me, they were planning a prank on one of their classmates that seemed fairly harmless but could be hurtful if the other student took offense. I confronted them about it the next day, explaining my concern. To my surprise, they were defensive and argued that there was no real harm in their plans. Despite my better judgment and knowing how upset it would make me look to enforce this rule so harshly, I grounded both of them for a week. Was I right or wrong to invade their privacy like this? Should I have taken more drastic action, or should I have let them handle the situation themselves and deal with the consequences if they occurred?