I haven't conducted any
research on the topic, so the below may, quite possibly, only apply to my school. For information, my school is a male-only
public school (in the UK sense) which has students from ages 11-18.
There are prefects at two levels in the school. At age 13-14, or upper-third form (the end of the lower half of the school) there were several junior prefects with limited powers, distinguished by a different tie. They existed mainly to vaguely keep order and assist at assemblies.
Secondly, there are the main type of prefects, appointed at the end of the lower sixth (effectively the upper sixth, because the year ends early when you have public exams). They are distinguished by a different tie, and have far more responsibility. They can give duty slips, which result in the offender having to assist in cleaning the dining hall and effectively losing a lunchtime. Prefects assist in monitoring lunch queues, assemblies and whatever the school needs them to do.
Within the (sixth form) prefects there exists a smaller hierarchy. The lead prefect is the School Captain (a similar concept to Head Boy). His deputies are the vice-captains. The most senior prefects are known as senior prefects.
Both junior and sixth-form prefects are appointed by the senior members of staff, with consultation from form tutors and at one point a vote by the students.
As well as the duties mentioned above, all prefects help with school events, which for a public school can be quite large. They assist with showing people in and around at concerts or sporting occasions, and this can involve a substantial amount of effort put in outside of normal school hours.
In addition to the different tie, at certain occasions prefects wear special gowns, blue for junior and purple for sixth-form prefects, with varying gold or black trim to show special status for the school captain and senior prefects. The sixth-form prefects attend a prefects' Christmas dinner in black tie as recognition for their efforts.
Finally, an additional class known as prefects are house prefects. They do not have any special powers but are appointed in the upper sixth to help in the organisation house activities, usually with a prefect assigned to each year. They are appointed by the relevant housemaster.