If you employ an external teacher to lead certain sessions, you can give them their own limited access login, so that they can easily view the registers for their weekly classes and mark attendance directly in your account.
Teachers do not have access to any personal data about your customers. They can only see the 'Print register' view - they can see the names of the adults and children attending the class, and the age of the children - as well as any important notes sent in by parents when booking.
To add a new teacher, go to Profile > Teachers. Here you'll be able to manage teacher access and choose which classes in your timetable they should have access to.
Note: Teachers will be able to see all of the registers for future dates for that weekly slot.