Sticking up for yourself is crucial, I learnt this quite quickly especially at school. Recognise what it is the other person is doing, and if they come at you, my advice to you is unleash whatever you have within you onto them- if it takes beating them then so be it. Have you ever considered boxing? This will give you confidence within yourself, it will teach you how to defend yourself and it will allow you to recognise you can handle yourself. Trust me on that one.
I would definitely recommend telling your teacher about how you feel, I didn't for so long and things just got worse. School was a battleground for me daily, I had no self worth and my anxiety was sky high. You need to tell people. Mental health is not a joke, nor should you take it lightly.
There are others out there suffering the same things as you, you are not alone. Set yourself a goal and work towards it. You also haven't explained exactly what it is you are going through, so it is a little hard to give any sort of advice.