Whether or not attendance submission "locks" after a teacher submits attendance is determined by the administrators at each school. Some schools require locking and other do not.
Schools who use locking only want teachers to submit attendance one time each period so that attendance cannot be updated/re-submitted. Such schools usually have an attendance administrator who is responsible for following up on all absent students to determine their whereabouts as soon as it becomes apparent that they are not in class.
Attendance locking and the whole attendance administration process is particularly import in residential schools where it's critical that staff know where students are at all times. For such schools to run smoothly, it's counterproductive for teachers to be allowed to submit and update and re-submit attendance repeatedly.
Therefore, these schools have a policy whereby the teacher takes attendance once and then attendance locks. Once all attendance is submitted the attendance administrator begins the job of tracking down missing children. If a student who was absent, then shows up late for class, the teacher can stop the administrator from searching for the student by simply calling or emailing them to tell them that the student is located.