Please bear in mind that the setting is an English school. If you don't know about the English education system (and the rest of Britain, for that matter), then you really do need to. This is not being favourist, it is simply so that we have common ground and not a place where half the staff call Year Nine 'Year Nine' and the other half call it something else.
You need to state one or two (three by permission) specific subjects that you want to teach. You cannot hover. Subjects in British schools include:
English (general GCSE level)
English Language (A-level)
English Literature (A-level)
Maths
Science (general GCSE level)
Physics (A-level)
Chemistry (A-level)
Biology (A-level)
Electronics (A-level)
French
German
Spanish
Russian
Italian
Art and Design (choose a specialism from Photography, Graphics, CAD or Fine Art)
Expressive Arts (required of Arts teachers)
Music
Drama (GCSE level)
Theatre Studies (A-level)
Media (GNVQ)
Business Studies
Economics
Sociology
Psychology
Religious Studies (general/GCSE/AS-Level)
Physical Education
History
Geography
Office Studies (GCSE)
Computing (A-level)
Health and Social Care (GNVQ)
Leisure and Tourism (GNVQ)
Business Studies (GNVQ)
Law
If you honestly think that you can think of something else then mail me and I'll see if it needs including.
You need to have a background. Think about what you would write on your CV if you were really applying for a job in this school. You also need to know details such as family, friends, etc. (but you can think these up as you go along).
You need to have a personality! Without it the game won't go very far. If you're stuck, base it on yourself or even on a teacher you once knew.
Decide on your appearance. Are you scary? Are you tall? Do you wear glasses? What sort of clothes do you wear?
All this should be the bare minimum for your character. You will find that the character develops the more you play them, and eventually takes on a life of its own.