How to lose weight without parents knowing?

Hi I'm a little overweight im male and i'm about 5'4 to 5'6 I reckon and my mum refuses to believe I'm overweight. My jeans are in husky sizes and I've got the things I cannot say i have to anyone but myself and appear on the chest that overweight men have cos i am a boy and my brother wanted to lose weight recently but she said no he was fine and she said the same when i was younger and my grandad is always saying how skinny I am but I know I'm not she hardly ever goes out so I can't do any exercise on my own without her hearing and I don't want to go running cos people will see and thats really embaressing whether people know me or not. I don't know how much I weigh cos the scales are broken and it would be really embaressing to go to some place where they have them. And my wiis broken so I cant look on wii fit :(. My Mum always gets things I'm not sure how many calories or how many to a serving but I want to look nicer for my next school disco in July (England btw dunno what it's called in America etc.) I can't ask for things with less calories cos she'll get really suspicious and she won't let me go out on my bike cos shes afraid I'll hurt myself anything I can do plz plz plz?