4 of them have been planning it for months - as soon as the exams finished they were off in an old mini following the route of some Audrey Hepburn movie ending up in Switzerland. It now transpires they are leaving the sunday before results and will be gone for 3 weeks 
So cross I can't even talk to him about it. I just threw the letter from school at him which has in bold "Students are strongly advised to be available in person to handle whatever situation may arise"
We are away on holiday until the Tuesday so I won't be here to stop him going - not that I can actually do anything although last night I had visions of going on holiday with his passport
His uni need conformation that he is accepting the place asap as it is only then that they start allocating the accommodation, its first come first serve and if he does nothing he'll end up with his 11th choice of accommodation which is a bunk room in a welsh medium hall and he can't speak welsh!
The other 3 on the trip aren't going to uni - they have jobs/apprentiships lined up so have no issue about being there to collect what ds says is "just a piece of paper". One is already working and can only take leave at a specified time which seems to be why they have picked to go then.
Don't feel able to make a decision should he miss a grade and be offered an alternative course. Why the hell is he being so irresponsible! So here is the dilemma
Option A -let him go he's 18 and if he forfeits his place at uni it is a lesson learned and he can apply again next year
Option B - let him go I collect his results and hope he gets the grades and make sure he gives me access to his emails so I can reply to any offer
Option C Don't let him go (consider again the idea of confiscating the passport) but get him a flight or a train ticket so he can catch them up on the Thursday eve or Friday