I am not an attorney, but the law is silent as to if school is in session or if the facility is being used for a non school event. Is does however says you MAY NOT carry on the premises of a school. Period To me it means just that 24x7. I think you would be a on a slippery slope if one choose to carry. Would you have an argument that the building was being leased for XYZ and not school, yada yada? Perhaps But an argument in court all comes down to what the man in the black robe feels like that day.
whether or not a school is in session does not change the fact that you are school property. It's not legal to carry a pistol into a school during summer vacation, neither is it on a saturday or sunday.