Mix and match different words. For instance, take a color you like, say 'Blue', and a high school activity like 'Drama', then you get 'BlueDramaForHSStory'.
Think about the kind of story you want to tell in high school. If it's about friendship, you could use 'FriendshipBuilderForHSStory'. You can also use alliterations. For example, 'FantasticFreshmanForHSStory' if you're a freshman. This makes the username catchy and easy to remember.
Look at the initials of your name and combine them with something from high school. If your name is Sarah Johnson and you love art, you could have 'SJArtForHSStory'. You can also play with words that have double meanings. For example, 'Sharp' can mean intelligent or an actual sharp object. So 'SharpThinkerForHSStory' would be an interesting username.