Tenancy Agreement Signed, Students now want to pull out

Tenancy Types and Management

I have recently taken on a student HMO to manage for a friend, due to Warwick University stepping away from off-site lettings management. The students in situ decided they wanted to stay for another year after me engaging with them at the start of this year. I recently chased them up and got them to sign up before going home for the summer, so I wasn't left in the lurch. 3 of the 4 signed up, the other one went home early without telling me.
The one who hadn't signed then let me know he wasn't signing up. That left the 3 who had signed the joint tenancy (the NRLA one) liable for the entire unless/until they could get another occupant, which I would help them with. The tenancy is due to start September 2021.

1. other have now pulled out, after having signed. Unfortunately this is before they have paid any deposit.

I think in reality there's little I can do, but would appreciate it any advice as they have wasted a lot of time for me and opportunity to find alternative tenants at this late stage.
Cheers