Schools or colleges can’t enforce people coming in for results day.
However, at the same time, they can and should strongly advise that people should come in and explain why that is.
Schools and colleges will help students who miss their grades and don’t get their uni choices. They can advise, show expert knowledge of processes and next steps and with a student there, can arrange priority re-marks that can be returned super-fast,meaning students might still get their uni place. Staff from different departments might be in, with grade boundaries who can advise if a student just missed a higher grade or was miles off. They can advise if review of marking is advisable or not, and if so, which paper. This kind of advice can be invaluable and make all the difference between a student securing their place at a uni they initially chose or now like the look of, and not.
Everyone should gather stuff ahead of results day for possible phone calls - UCAS details, copy of PS, phone numbers of different places etc. clearing moves fast and the prepared and well-advised do better in it.
Being on holiday or at work and not going in, simply potentially makes students disadvantaged if things do t work out quite as planned. So no, schools and colleges can’t enforce it, but they should recommend it and see it as part of their final service to their students as providing the vital support some will need. To be honest, putting yourself in a position where you can’t avail yourself of this potentially vital support is pretty daft, especially when you’ve worked so hard for your results.