Failing To Obey Sign (do Not Enter)

I just got a 110 buck ticket today for failing to obey a red circle with a white bar through it sign, that I didn't see. The street in front of our high school here in Brighton has been partially closed for weeks, but you could still go down it and if there was no oncoming traffic proceed around the barriers in a single lane, until it went back to 2 lanes. Well I guess that changed this week, and they have the whole southbound lane closed. I was heading west on another street and there was a van signalling to turn left on my street, so I stopped at the stop sign, signalled to turn left and let the van proceed. I then made the turn into the single lane and proceeded about 100 feet to the school parking lot, then went through the parking lot to get to the exit to highway 2 where I wanted to go. I didn't get too far. A cop must have been hiding somewhere watching, and he was soon right upon me with his lights flashing. I was still in the school parking lot. He proceeds to inform me of the do not enter sign and tell me that he was an off duty cop working for the town, to specifically guard the entrance to this street and blah blah blah, no matter how much I apologized he stuck me with a 110 buck ticket that I see carries 2 demerit points. I probably don't have a leg to stand on, but the thought of those 2 demerit points on a clean driving record irks me!

Any clues on how to proceed would be appreciated!

Thanx!