Are you copying or are you creating your own school building? If you're drawing an existing building, simply go to the school or find a picture of it. Find a decent place to copy the building. Make sure you're comfortable as you may stay for an extended period of time. Remember that you don't have to be in front of the school. You can always draw at an angle. Have your materials ready with you like sketch pad/paper, pencils, and pens. If you're creating a new one, the first thing you can do is look at some pictures of schools online. This can inspire you on what your building will be like. Have the necessary materials ready when inspiration hits you, like papers, pencils, and pens. Sky is the limit!