Some schools have classrooms that are covered by a number of teachers at different times. This is known as a job share and usually it is two teachers splitting the days of hours of the day that they spend teaching in the classroom. It may be that the head teacher job shares with another member of staff allowing them time to complete the other responsibilities they have or that you have two part time staff looking after one class.
Working as part of a job share can have advantages as well as disadvantages both of which should be consider prior to taking this type of role. If you are sharing a class with someone you may not have as much control over how and when you teach certain subjects. You may also need to reply on the other teaching performing well to ensure that the pupils are educated to the level they need to be. With a job share though, you may have more flexibility allowing you to occasionally swap days round if you need time off.