When i was caught 1st time, (cuz my dad said my room smelled like smoked) they didn't do much and just kept sayin shit like "why does it smell like smoke in here?" and i had to keep comin up with lame excuses like "I burnt sum paper cuz my friend said the flame went green" (or something like that).
Then eventually my mum and dad came bustin into my room 1 sunday morning and woke me up and accused me of smoking and said i didn't look "all there". But nevermind, and then my dad found my pipe in my cupboard (searching through ) and came back in my room later demanding i give him my pipe, but i said no and hid it under my bed (I was planning on sneaking out of my window in the middle of the night and throwing it somewhere, so he couldn't get it off me) and it cost £8 so i thought "fuck that idea" and kept it and just kept refusing to give it to him

Then up until about 2/3 months ago i would just get the odd "your room smells like something has been burning again". so i just ignored the remarks and carried on smokin', but this time in my downstairs toilet (bathroom for US ) and it's only like 5x3ft with a toilet, sink and extraction fan, so i just do it in there now, and i know my room smells like weed (as even i can smell it, but luckily my mum is ill so she can't smell anything ) and they haven't mentioned it since....... The End

So in conclusion i would say, don't tell them in case you get in trouble about it, and what they don't know doesn't hurt 'em (or something along those lines)