If you missed a few people and want to add them to the record of the event, you can add a whole class of people at one time.
From the Check-Ins tab on the event, select Add check-ins to add multiple people at one time.
Make sure you're on the right date by using the arrows or choosing a date from the calendar.
Choose the location, time, and type of attendee the person was then type their name in the box.
Any warning messages you might have seen in Station mode will show underneath each person.
Once you've added all the people to this list, select Check In x People, and they will be added to the check-ins for that event.