As soon as you say that you are a state school teacher, everyone looks at you knowingly and starts discussing the appalling behaviour of children nowadays. Everyone thinks they know what it is like to be a teacher, after all, everyone knew one once. But things have changed. Same place, same children, same staff, but not the same job and never dull. Add in the ever-changing bureaucracy and the constant need to reinvent yourself / your teaching / the clothes you wear that you think are quite cool till the kids tell you otherwise, and you walk out of school at the end of the day feeling as though you've run a marathon!
A PGCE qualification in your chosen subject is a must, although some degree of flexibility is needed with regard to the teaching of PSHCE - that's 'sex, drugs and rock and roll' with a side helping of Citizenship for the uninitiated!
What you need perhaps most of all is to like children and to want to see them achieve, no matter their background or their definition of success. A considerable number of the children may well not be able to access the same level of academic thought as you, so patience, enthusiasm and the ability to give clear explanations are essential. You need to be able to laugh - with others and at yourself; you may once have thought that you were quite cool, but it won't take you long to realise that you just aren't!!
Other than these particular skills, you need the skills you would expect from a job with people - good communication and organisation skills, a positive outlook, tact and diplomacy and a desire to develop and nurture a child's mind and horizons.
Teaching is hard - the hours are long, maybe not always at school, but certainly at home, the children can get to you in a way that adults cannot, and being at the chalk face means submitting to the whims of a government or senior management team. The rewards are great - the feeling of doing something worthwhile, the smile through the tears of a child who finally feels they are being listened to, the 'thank you' at the end of 5 years from a particularly challenging child whom you actually managed to get to sit a GCSE in your subject, the boy who is the first person in his family to go to university, let alone Oxford or Cambridge, the girl who has managed to get a couple of GCSEs despite having a 4 month old baby and the quick 'see ya miss / sir, I'll never forget you' as they walk out of the door and out of your life. Of course, the 13 weeks of holiday is also an obvious plus.
Career-wise there are certainly places to go, but at the end of the day, there is no point working in schools if your heart isn't in the children and the teaching. Good school management is obviously crucial, but actually the whole "raison d'être" is, and must always be, the needs of the children you teach.