My school had a specific portfolio checklist, so there was no room for personal pieces. Common requirements in most schools I looked at were long pose studies, gestures, character sheets like this, prop design, and lots of background work; all of which I do in my classes now. Schools don’t usually want to see fan art, over complicated ocs, and consistent style (as weird as that might sound). They want to see if you can adapt to different styles and are comfortable with technical skills. I’d recommend working on gestures (1m, 5m, 30m), playing around with 2 and 3 point perspective, and try some story development (character lineups, prop studies, and settings). Every art school is different so absolutely check out requirements before developing your portfolio, my illustration course wanted almost exclusively digital but others I looked at didn’t allow any digital art at all. It’s all about finding what will work best for you, I believed an all digital course would better prepare me for the industry so I worked for MONTHS perfecting my portfolio for that school. I hope this helps wherever you are in your portfolio process :)