Tell them. Look them in the eye squarely and tell them. If they won't budge, you have to do what you have to do to drive your life. You don't want to grow up one day and realise you life has been ruined by your parents do you?
My strict-as-hell parents promised to let me go for a singing course only after I pleaded for three months continuously. Today they said no again. Well, guess what? I got so sick of taking no for an answer I just emailed the singing school and informed them that I'm interested.
I'm working soon so I guess I could pay for the course. I might ask my parents, but if they say no I am seriously stealing MY bankbook from my mum's closet and whisking it off to the bank. Funny right? I have MY own bank account with MY money that I saved up, and I cannot touch a single cent of it according to them.