I'm not aware of any public schools in my area that would allow me to, e.g., use the basketball court or soccer field after school hours or on the weekends.
There is one school that definitely is gated off, but that's because it's near a major point of interest and I can only assume they're worried about non-community members damaging the property.
Around here the grounds are not only open outside of school hours, but explicitly so (they have closing hours posted: 9PM).