Not got anyone doing exams here (apart from uni ones haha) but thought I would flag a bit of how it works...
(This assumes an application to uni)
UCAS portal updates at 8am so they can look then and see where they have got in. This doesn't tell grades but tells them if firm choice or insurance has accepted them.
They can then go in to school (or log on) to find their actual grades, which might be higher than hoped or lower (mate of DD's got in to top choice even tho he had much lower grades than offer).
The thing with the UCAS portal is that they know which way it is going to go, so it's not so shocking (as they have found out the main detail at home).
DD missed both her choices and was v upset - but we were at home. She then went in and found her actual grades - I joined her and we did the clearing process together. My main advice with clearing is to be realistic about it - you may need to lower your sights. DD was targeting RG unis but in the end her grades got her into Uni of Leicester, which actually suited her better in the end. It was a stressful day tho (understatement).
DS2 knew at home that he had got in to his first choice - went to school and found his actual grades. All good. Tho after DD I had taken the whole day off work - I would advise this if there is any likelihood of your DC needing to do clearing. At numerous points we and her head of 6th form were all on the phone to various unis! Good luck all.