Why the Cullens Go to Highschool...At Least One Way to Look at It
Sources
The Twilight Guide
The first Twilight novel
The Cullens have already gone to college. Several times. They all have degrees except Emmett--who apparently isn't that into learning. Edward and Carlisle have the most degrees. Edward, though already having gone to college many times before, wants/planned to go to college with Bella after she turned. Going to college is already one of the disguises they use and high school is just one of the ways they do, but they seem to prefer starting to go to highschool.
(headcanon) I imagine they like to mix it up, just out of the need to stave off boredom.
The Cullens, especially Rosalie, want to stay in a particular place as long as possible. Being teenagers in high school allows them to do so.
(headcanon) Highschool is one of the current human experiences and the Cullens (Rosalie, and, for different reasons, Emmett, Alice, or Jasper) appear to want to feel as human as possible. Since they aren’t immortal children, the earliest experience of being a human that they have access to in this modern age in the U.S. is teenagerdom/high school.
(headcanon) Teenagers are more likely than adults paying to go to school to ignore weirdness or pass it off as such. Not because they are stupid but because they don’t want to involve adults in most things--many times justifiably.
(headcanon) The Cullens and any vampire posing as a human family for whatever reason can use human teens to understand the zeitgeist of the time so they better mask their public activities. Better, since teenagers tend to be the ones trendsetting or at least more knowledgable and passionate about trends. Teens also come up with a lot of new ways of thinking and criticisms of how adults do things because they are trying g to define who they are. College kids can be trendsetters and self seekers as well, but they're more tired and focused on building careers so they, in comparison to "average" teens are less on the uptake or less able to devote themselves to understanding social trends unless it concerns their major/future career.