Personally I think you're making a stupid decision but hey, it's your life. Sit down and calmly talk to them about how you're going to be an adult and it's time to make your own decisions then slowly work your way into the point: I'm going to be living with [name of boyfriend] for a while until I am able to fully support myself and live on my own.
The easiest way to allow your plans to unfold is not necessary coldly 'telling' them facts .. You may soften the blow by opening the topic up for discussion. You may have to convince them that you know what you are doing and that you are prepared.
My guess is .. Breaking ties abruptly will not go over well .. if you don't want the drama then let them be involved with your life and your plans even AFTER school is out. Don't forget, they will always be your parents .. and you will ALWAYS need them on many levels. Don't just slap them in the face with it .. let them be a part of it.
Then every one is happy .. and they may even help you move!
Don't allow others to rush you into a decision. (Proverbs 29:20) Even if you have valid reasons for leaving home, you'll need more than good intentions. You'll need survival skills-money management, domestic skills, social skills, and personal spiritual routine. Do you want to leave home to get away from problems? Or to break free from parental authority? If so, your focus is on what you're leaving, not on where you're going. That approach is like trying to drive with your eyes fixed on the rear view mirror-you're so preoccupied with what you're moving away from hat you're blind to what is ahead. Don't just concentrate on moving away from home-have your eyes fixed on a worthwhile goal. Whatever your goal may be, think it through. "The plans of the diligent one surely make for advantage," states a Bible proverb, " but everyone that is hasty surely heads for want."(Proverbs 21:5) Listen to your parents' advice. (Proverbs 23:22) Pray about the matter. The real question is not Am I ready to leave home? but Am I ready to manage my own household?
Let them know in advance. Mum & Dad you both know how much I love you & am grateful for all you have done for me. I'm needing to find my feet & independence & am thinking that I might try my luck out in the world of independent living. I would like to move in with my boyfriend & I was Hopeing we could talk about it as I love you both dearly & don't want you to feel like your loosing me cos your not. I am & always will be part of our family & think I need to experience life supporting myself & being a mature & responsible adult. Listen back to what they have to say. Then one by one relieve each worry that they may have by explaining how you will overcome the obstacles that you will face. Think out your answers to obvious comments like how will you pay the bills. Do you know what current rental costs are. Do you know how to do a house hold budget etc so they see you have thought out & planned this discision. They may think you are rushing in but if you show them you have preplanned, then that may cause them less worry.