If you exited the Mondrian parking garage on Blackburn and turned right on to McKinney, then you would not have seen the flashing light that's before that exit. There's also a flashing light on the other side of Blackburn, before you get to the intersection, which you would not have seen either. I think you have a solid case. Start by going down to where you would pay the ticket and tell them that instead you want to set up a jury trial, and make arrangements to contact the prosecutor. You'll need proof you live in the Mondrian, that'll establish the reasonable assumption that you turned on to Blackburn after the light, and also take pictures of all the school zone signs from Blackburn to the end of the school zone, there are two that I found on streetview. You'll present this to the prosecutor as basically the case you'll pursue in front of a jury, and the lack of flashing lights on the signs makes them largely unenforceable since you couldn't see any flashing lights starting from when you entered the roadway on Blackburn. With any justice the prosecutor will dismiss the ticket outright because it'll be hard to prove to a jury that a lack of visible lights and a lack of printed school zone hours on the signs isn't enough to find against the city.

I would also seriously consider getting a dashcam. Video evidence is always strong to a jury, so if you'd had video of exiting your garage, turning, and driving with no visible flashing lights no jury would convict you and no prosecutor would bother bringing a case. The only fix the city has is to add flashing lights on the east corner of Blackburn so that anyone turning onto McKinney would see them.