Is there an iphone app that prevents procrastination?

I want an app that blocks websites i choose for a specific amount of time so i can get more work done in school. I have something that does this for my llaptop that prevents me from web browsing but is there anything on the app store that could do the same?

thanks

Added (1). alien - because I can just turn it back on? I want something that forces me not to be able to web browse for a certain amount of time.

i asked for apps, not alternatives.

Why not just turn off the phone?