Your CPL instructor needs to review the law again, I'm afraid. The statutory school property restrictions on guns make no reference to hours of operation or seasons. They merely define the restricted zone (a school) and the fact that parking lots and drop-off areas are exempted from the restricted area when one is picking up or dropping off a child "from the school."
Perhaps it would have better to state that the manner in which it is worded could leave room for debate on whether after school events qualify as picking up or dropping off "from the school." It probably is a good point to make since over-zealous "authorities" might try to use this to further limit our rights.