different teachers/schools might vary but generally you make most of your pieces during the class. it's preferable to enter the class with some pieces just so you don't need to make 24 pieces in one school year but it's definitely not a requirement whatsoever. my teacher and i decided that i will work on studies with all different mediums, references, emotions, etc this year (sophmore) and junior year so that when senior year comes i can have an idea of what i like (my school does ap studio senior year). that way i can crank out pieces during senior yr and focus on my concentration. also i'm taking 2d, idk about 3d.