no-turn signs are often erected to control traffic volume on residential streets. If I "guessed" your scenario with the description above, I don't see how you can beat it, although I believe a sign like this needs to be enforced by a City By-Law. You could take the chance that the Crown won't have the official By-Law at trial, and motion for dismissal on those grounds. The Crown may motion for a continuance to get the necessary papers. Object fiercely because you have already lost time from school, and you don't want to mis more... blah, blah, blah.
I'm not real sure bout this, so you might want to get further input.
I showed up in court on both cases, cops were there but charges are withdrawn. Need more details, send me your email address.