UC says in http://admission.universityofcalifornia.edu/transfer/index.html that “You’re a transfer student if you enrolled in a regular session (fall, winter or spring) at a college or university after high school. (Taking a class or two during the summer term immediately following high school graduation doesn’t make you a transfer student.)”
JHU says in https://apply.jhu.edu/application-process/transfer-students/ that “High school students currently in a dual-enrollment program should apply as first-year students.”
Harvard says in https://college.harvard.edu/admissions/application-process/transferring-harvard-college/transfer-eligibility that “Students who have completed one full-time year of college in a regular degree program in lieu of their senior year of high school (often referred to as dual enrollment) should apply for first-year admission if these courses are taken for credit towards a high school diploma.”
For other colleges, investigate carefully. In general, taking college courses after high school graduation is the type of action that could force you to apply as transfer instead of frosh as many colleges. But each college may have its own special rules.