Rent Recovery through Legal Process

Some of these are rented out to students . I have a joint Assured Short hold Tenancy Agreement wherein though the students are not related to each other have agreed to a joint and several liability for rents.
Now one of the students has left halfway and left to study in a different University and is refusing to pay due rents as she cannot afford to pay at two places.
I am duty bound to recover the same on behalf of the Landlord
I wish to ascertain the legal situation and take necessary action.