This is an interesting one!
If your g/f contributes to the bills of the house in any way and is using the address for correspondence, then she is already deemed to be living there. She has to prove that she is 'just visiting' and that she has her own normal place of residence, to be your 'guest'.
The old standard used to be that if she stayed with you for more than 3 or 4 nights a week (I forget which it was now) then that made her a resident of the household.
You have an obligation to inform your landlord of the additional tenant, as nowadays, each tenant should be named on the tenancy agreement.
Also, as you are a student and she is working, you must also remember that If you are a student living with your partner, your partner's income will be taken into account when deciding if you are entitled to a student grant.
And finally, you should also inform any benefits bodies, and the local council, for council tax purposes and electoral roll purposes as well.