In trying to answer your question MissA*, I managaed to find this.
S T A T U T O R Y I N S T R U M E N T S
2006 No. 1751
EDUCATION, ENGLAND
The Education (Pupil Registration) (England) Regulations 2006
Made - - - - 1st July 2006
Laid before Parliament 11th July 2006
Coming into force - - 1st September 2006
In exercise of the powers conferred by sections 434(1), (3) and (4), 551(1) and 569 of the
Education Act 1996(
a), the Secretary of State for Education and Skills makes the following
Regulations:
Citation, commencement and application
1.
—(1) These Regulations may be cited as the Education (Pupil Registration) (England)
Regulations 2006 and come into force on 1st September 2006.
(2) These Regulations apply only in relation to schools in England.
In these Regulations —
“the local education authority” means, in the case of a school maintained by a local education
authority that local education authority, and in every other case the local education authority
for the area in which the school is situated;
“maintained school” means a school maintained by a local education authority;
“relevant person” means —
(a) in relation to a pupil under the age of 18, a parent of the pupil;
(b) in relation to a pupil who has attained that age, the pupil.
which appears to say that a pupil under 18 has to attend and be registered.
Contents of Attendance Register
6.
—(1) The following particulars must be recorded in the attendance register at the
commencement of each morning session and once during each afternoon session —
which explains the sixth forms request she attend all day.
The full text is here and is not interesting reading.
Maybe have a read of it with your sis and parents before the meeting 'cos I imagine the head will be able to quote it!
Pete.