If a large number of students were signed in by mistake, you can remove the attendance data from the Student Sign-In screen of the website.
Click on the white check box next to the students' names, or use the checkbox at the very top to select all of the students at once.
Click on "Select Batch," then select the "Batch Delete" option.
Press the "Delete" button and confirm your action.
This action will only delete the students who are viewable on the screen, so you will need to repeat this process if you have more than 40 students.