Is using a school football field at night illegal?

[everlastes]

I use a football field at a middle school by my house to jog and run sprints 2 days out of the week. I usually go around 8 pm because I go to school and work during the day, so its usually dark when I go. When I go, all I have is my ipod and my car keys on me. The gate that leads to the field is usually locked, but there is a hole in the fence where you can still pass through. I saw some cops around the school after running today and figured someone told on me, and I was wondering if I'm doing anything wrong? I just go in, jog and run sprints, and then leave. As a taxpayer and a former student of the school, I dont think I'm doing any harm, but laws I dont know about might say otherwise.

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