You can ask the school administration. My old high school has an indoor track in the new gym they built when I was in junior high (so not so new now). It's open to the public during the day. You just go in and ask. It's a small town so they generally know everyone. I went in there once when I was home on leave.

The track at our football field isn't ever locked and it's open. I went there and I used it before I went to basic so I could get in shape. No one said anything but again small town.

Best bet is to go ask and they will either tell you yes or no. If it's no, go find some other area to run in or join a 24 hour gym - or buy your own equipment.