UCAS states that the aim of the reference is to ‘give universities and colleges an informed and academic assessment of an applicant’s suitability for further study’. The reference is a key part of a student’s application so it is important to gather the relevant information to write this. It can be helpful to consult the students subject teachers who will be able to provide information on their ability in and attitude towards that subject. In addition to discussing academic capability it is also important to talk about the student as a whole so do include details of any extracurricular activities and personal attributes that will help them succeed on their chosen course.
The UCAS site has some helpful guidance on what they want from references and what information you might want to include: