Interim agreement for student summer period

Tenancy Types and Management

Hi everyone, I wonder if you can help? I have a student house with two long-term very responsible healthcare students who are renewing their tenancies for the next academic year. One of the students will be continuing to live at the house continuously on current terms over the summer as she is working on placement in our local hospital throughout, but I'm allowing student#2 to leave her belongings there and pay a reduced rent while she travels over the summer period. Can anyone suggest what sort of agreement I need to set up with student#2 for the in between period while she retains her room between her current tenancy ending on 16 June and new tenancy beginning on 30 September?