About half of all schools tanner stage girls each year.
Your school will tanner stage you to ensure you are developing from year to year and to assign you to sports teams with other girls at the same stage as you. A tanner stage number refers to how developed your body is sexually. In turn, that relates to your overall physical strength.
The fairest way to match up sports teams is to ensure that all of the team members are at the same tanner stag and that they play other teams at the same stage.
Tanner staging is normally done as part of your phys ed class at the beginning of each year. Typically its done in the girls locker room. You can be tanner staged while wearing just bra and panties – if your school doesn’t require communal showers this is common.
In schools that do require girls to participate in communal showers, a more accurate tanner staging is done when you are naked. Part of the staging process involves to extent of your pubic hair growth. This can only be accurately measured when you are naked.
In either case you will stand in a line with the other girls and a female coach will look at your body. She will assign you a number based on your development. You can find out more about tanner staging here.