Taxes as a freelance writer and student?

I am a student (from the UK) I have begun writing on Textbroker to earn some extra cash, but as it's pretty much freelance writing, the site says I need to declare myself self-employed (I am not employed by them), but I'm confused as to how it will affect my tax. I am only doing this in my spare time and expect to earn around £1500 so obviously I will need not be taxed, but I still need to register with them. I will also not have to pay national insurance contributions. However, this is not my only income. I am emloyed in a part-time job as well- do I need to tell them this? Also, when I leave university, I hope to have secured a teaching post (I study Primary Teaching), so what happens then, because then I will earn enough to pay tax and national insurance, but will that be automatically be deducted from my wages, and can I not pay tax on my freelance writing at the same time? On the site where you register to be self employed and state that you have low earnings, it says that it can affect basic state pension bereavement benefits and maternity allowance. I do not what this to be affected when I am a full-time teacher but writing at the same time. I am just confused, somebody help me!