Hello I currently go to a catholic high school and taking 2 honors and a accelerate course along with one AP. I am also making my confirmation this year and missing a day of school, I don’t want to miss school since I feel like I won’t be able to catch up and fall behind. Please HELP I don’t want to miss a day of school so how do I tell my parents no 
You don’t. Talk to your teachers ahead of time to let them know. Make an appointment to see them with the next day or two after the day off if you need help with the material. Ask a friend to take notes during class. If one day tanks your grade, you didn’t have a handle on the material anyway.
People get sick all the time and miss a day of school. You can catch up easily if you miss a single day.