There are 2 cases here.
1) Where you are the one who signs the contract - If this is the case then read your contract carefully. The contract might prohibit you from subletting it to others. If not you can sublet, but I would make sure that the landlord knows about it and gives his/her permission. They might want to draw up a new contract. Or if you sublet without them knowing, you know not what they might do.
But everything is decided by the contract you signed. If you had sublet without the landlord's permission, and the contract states you aren't allowed to do so, then the landlord has full rights to ask you to vacate the house as you have broken your contract, but he/she needs to give you the notice period mentioned in the contract. If all is well but the other tenant broke or destroyed something, you are the one who has to make it good. Remember your neighbours might inform your landlord that more than 1 persons are living and he/she might know even if you didn't inform the landlord.
2) Where both of you sign the contract - Everything is decided by the contract you both signed i.e. bills, making good anything broken or destroyed etc.
In your case if the student is your family member/friend make it clear to the landlord that both of you will stay and the contract will be drawn accordingly. Make it clear who is paying the rent, so the landlord knows. The landlord might want to do a credit check or see your salary slips and all. In that case you need to sign the agreement and the other person doesn't need to. If you are subletting then it is a different matter.