My high school was based around this idea of student independence and was set up a lot more like a post-secondary institution than many high schools (probably better "college prep" than a lot of hyper restrictive cram institutions that advertise themselves as such), and we had a testing center! Things were all set up for "go at your own pace", so they couldn't be having subject teachers proctoring exams whenever one kid got to that point in the course, so instead they had a centralized testing center where you could book an appointment ahead, or walk in anytime prior to the final hour of the day. It was great. Tests were held in one specific place, which was really helpful for getting into the right mindset to take a test.