Ambush him when he comes out ofthe bathroom; that's what my cousins often did to me when I was a teenager. Of course there were two of them and they were older and more athletic than I was, which helped. Or if you're especialy desperate ambush him while he's still
in the bathroom. Nobody's ready for a fight when they're using the john... but this might be fighting dirty - literally - and requires careful timing for reasons I trust are too obvious to need listing.

Dare to be different... and make a difference.
To boldly go where no one in their right mind has gone before...