Well when I wanted to watch a showw that was considered "Adult" when I was 12 (19 now) called "Queer as Folk" I counted mostly on my Mother falling asleep at her regular time, I became AWESOME at changing the channel at the mere thought of her coming near my door and I was lucky. We had all of the cable channels, so Id's stay up late (It came on Showtime) and watch it on Showtime West, it came on 3 hours later, but I loved it.
Although after a while, I got tired of having to sneak it and just began to watch it whether my Mom came in or not. As long as I was doing and keeping up my end of the deal, I felt I could watch the show. Plus, I was much more mature than alot of my fellow adolecents. I eventually ended up talking to her about it and even though she didn't agree, she let me watch.
It WAS a challenge though.
But I feel like you're not talking about something as scandalous as "Queer As Folk" and oyu sound mature, so I don't understand the problem. You're not pregant, right? Talk to your parent's, highlight your strengths (maturity, homework being done, to school ontime, etc) and try to listen to them. If no budgin, then I suggest you get real good at changing the channel.
Also you could try Downloading the Videos online and watching it on the computer if that is not monitored as much as T.V
Try out
MegaVideo.com
Surfthechannel.com
Veoh.com
or LiveVideo.com
To Download, download the latest version of RealPlayer, and Firefox. And whenever you want to D/L a video just put your mouse on the video and in the upper right hand corner a box will pop up asking you if you want to D/L the video. Very cool thing. Helped me out alot.