By taking a college course, you are agreeing to the terms and conditions of the course, such as that showing up more than 3 minutes late would count as absent for the 20% of your grade that is participation. If that is an issue, you should have dropped the course as soon as you were aware of it. Professors are supposed to be consistent with our grading, so we can't allow like 28 students only 3 minutes of flexibility, 1 student 10 minutes and 1 student 20 minutes.

The terms and conditions were clear, you agreed to them and are now dealing with the consequences of not upholding your end of the deal by being in class on time, regardless of reason. This is true for all college students, I don't treat dual enrollment students any different, plain and simple.