Holding deposits

Tenancy Deposits

My daughter has a house of her own at Uni that she is letting out next year. Students source their accommodation now for next September. She has now found suitable tenants who wish to secure the property for next year and indeed she wishes to do the same so as to not have prospective tenants who later change their minds and leave her with a potentially empty property. She is therefore asking each of them to lodge a "deposit" but they will not be signing a tenancy agreement till much nearer the time.
My question is therefore, does she need to protect the "deposit" now or is she only required to do this once the tenancy agreement is signed?