This is a tricky one to give a straight forward answer to because I will need a bit of additional information to know for certain how far around one lap around your school's soccer field is. This is because, depending on how big the soccer complex is, the distances might vary. At a lot of schools (especially grade schools, high schools, and smaller colleges), the school's track and field is located around the soccer field - meaning that the track circles around the field itself. If that is the case at your school, then running around the field is about a fourth of a mile because one lap around a standard sized track equals 400 meters (and 1600 meters equals a mile).
However, at larger schools, colleges, and universities that have bigger and more prominent sports programs, the soccer field will have its own complex - as will the football field, the track, and most of the other sports. If this is the case, and your soccer field stands alone, then if you are running around it, it will likely be a bit less than 400 meters. You can estimate that it might be closer to 300 or 350 meters. If you really want to know how far one lap around your soccer field is, try wearing a GPS watch. GPS watches track your distance and mileage, so you can use it to track how far you have ran. This will help you and your friends know how far you run when you run one lap around the soccer field, which will help you figure out how many laps you need to make to run a mile, or however long of a distance you are trying to run in total. GPS watches can be expensive, so you might need to start budgeting for one. But if running is something you are interested in getting serious about, then a GPS watch will really come in handy! It not only tracks distance and mileage, but it will tell you how fast your running pace is, how many calories you have burned, and most will even sync up to your phone so that you can have all of your information and metrics readily available whenever you want to look at them.