Prank videos, inappropriate posts, rude "jokes", etc. have all caused high school students to lose scholarships, and have even caused colleges to change their decisions about accepting students' applications to even attend! Think about what you post BEFORE you post it. There are search engines that can find deleted or archived websites, what you post may still be accessible after you delete it, costing you opportunities for years.
Back in my day, students would send notes around when we got bored in class. Little, intricately folded bits of paper, sailing around the room when the teacher was writing on the board. We all knew we ran the risk of the note being intercepted and possibly read aloud to the whole class, but, it was what it was. Today, we tend to turn to our phones or other devices when we need a little entertainment. We feel like our devices are more secure, but they aren't really much better than a paper note. People screengrab or copy pictures and post them in other places all the time! A good rule of thumb is to practice not saying anything online that you wouldn't say to someone's face or to a large group. If you want to keep something private, don't say it online.