It should be clear in their separate deed of guarantee that they are 'jointly and severally' responsible. Do not make any assumptions!
For example, include in their deed of guarantee:
'In consideration of your granting a Tenancy of (property address) to (one of the student tenants), who will be party to a joint tenancy, I hereby agree to indemnify you against any default in payment of Rent, *mesne profits, damages for use and occupation, damages for disrepair, losses, cleaning of the premises to the original standard, other reasonable damages and costs you may incur as a result of the said tenancy, or any further occupation of the property after the expiry of the tenancy whether under a statutory continuation thereof or not.
*Mense Profits. These are monies that may be demanded from an occupier who remains in the property after a tenancy has ended. They compensate the landlord for loss of rental income but are nor rent, so do not create a fresh tenancy.
Include in the student tenancy agreement under the 'Definitions and Interpretation' section:
'Guarantor' is the person responsible for discharging the Tenant's obligations if the Tenant defaults whether the Landlord elects to pursue the Tenant or not. The Guarantor is the person responsible who Guarantees payment of Rent and other obligations of this Tenancy under a Deed of Guarantee. The Guarantor is liable to pay all Rent and any debt arising from any breach of the Tenancy Agreement until all debt is paid in full. For example, if the Tenant doesn't pay the Rent, clean the Premises to the standard outlined in the Inventory and Schedule of Condition, remove rubbish or pay for damage or losses then the Guarantor will be liable to pay. Guarantors are over 18, UK homeowners with a regular income, which is sufficient to meet their potential liabilities.
Also include a clause in the tenancy agreement:
'The Tenant and the Landlord acknowledge that the Landlord has entered into a separate Deed of Guarantee with the Guarantors, under the terms of which the Landlord will seek to recover any outstanding monies owed should they not be paid by the Tenant in accordance with the Terms in this Tenancy Agreement.'
The student tenancy contract must always be dated after the dates on the separate guarantor agreements, otherwise the guarantor agreements will not be valid.
Send each guarantor two copies of the student tenancy agreement. They retain one copy and return the other signed and dated across the front. This is useful if there is an issue later and they claim they 'didn't know' or 'hadn't seen the tenancy agreement'.
If the guarantor, usually a parent, isn't prepared to accept this 'joint and several' responsibility suggest that your future student tenant finds another guarantor.
It is dangerous to make any assumptions in the (student) landlord business about guarantor liabilities!