Complex sentence: One of the incorrect thing is that Kathryn and Sebastian are Sophomores but it takes place the school year before the first movie in that case they are supposed to be High School Juniors because int he first movie they are about to enter their senior year.
Split into simpler sentences: One of the incorrect things is that Kathryn and Sebastian are Sophomores, but it takes place the school year before the first movie. In that case they are supposed to be High School Juniors, because in the first movie they are about to enter their senior year.