Thank you for the question. Its encouraging to know that you are willing to open up to your parents with matters that concern your life. I would suggest that you should politely talk to them in a calm tone.
Be confident and demonstrate that you know what you want and that you are serious about your decision. For example, if your parents want you to be a day student but you want to be a boarder, show them the benefits of going to boarding school.
On the other hand, allow them to share their views so you can understand their point of view. This also shows that your respect what they have to say. I believe that you and your parents will find a way to make things work