Student contract signedm, can they withdraw before start date?

Hi all

I have a situation whereby a student group have signed up to an AST several months before the start date of the agreement. They have paid the deposit and all paperwork has been completed. Now one of the group has decided to drop out of the group and does not want to move in anymore. Are the tenants able to withdraw from the contract as a whole as it technically has not started yet? I understand that in usual cicumstances a replacement would need to be found but wasnt sure if as the start date was in the future could they all just walk away even though a ocontract was signed and deposit paid?

Thanks for any help you can give.