I think a High School EP could be fun, it will depend a lot on how in depth they would like to go with it. The challenging part is balancing studies and fun, and with teens there is probably other things that they also are occupied with like trying out different activities that they like, finding friends, joining cliques and finding someone to go to prom with. There could be larger events happening at the high school (like festivals) where you sim could attend and improve their grades in different areas, like a sports event, a science event and a musical and art event.
They could expand the study mechanic from DU and have it involve several steps, active classes could be more active and require more of you to move and do tasks, music classes, art classes, math classes, physics and chemistry classes. Maybe your sim needs to have good needs while attending class or you have to study more in your free time, you could have goals or tasks that you would need to accomplish.
As for other activities, there could be things like sleepovers, learning to drive, skating, baseball and cheerleading.
They could also make it possible for you to build your own high school, you could build an old high school full of mysteries and magical things or an ultra modern high school with with high tech solutions.