So there you are then.You say you get stressed at school and prolonged stress will heighten anxiety and nervousness. I was a very nervous kid myself and one byproduct of this is that you internalise a lot of what is going on, lip picking/biting is a byproduct of this internalisation. School is a tiny part of life, in fact it's not all that important at all. Try to take time for yourself, relax more, try to figure out what it is that is stressing you out so much and take steps to address this. A good step to take would be to eliminate refined sugars in processed/junk foods, and try to increase your intake of fruit and vegetables.
It sounds as though you have the beginning stages of exfoliative chelitits, make sure not to peel ot bite them from now on. If you must, purchase some aquaphor and keep the lips constantly moistened, see daniel miller's blog.