The problem is, as noted before, you've already admitted you're aware of the GFZ. Now, the fact that your typical entrance(s) not being posted might have been a valid defense if you can make a judge and jury believe you've never used any other entrance or exit in the school grounds.
Plus, I don't know about this school, but mine has a "Student Code of Conduct", and it mentions firearms. It's bundled with the school map and schedule - which are used when registering. So a jury could say that it was reasonable for me to have seen it (thankfully, CO allows firearms by valid CHP holders on public schools).
The point is, really, that you couldn't point to a non-posted entrance and say "This GFZ is invalid!" You'll still be prosecuted, and that'll be your only defense. It's up to a judge and/or jury whether your interpretation is valid.