I remember at my sixth form college you had to press your ID card on the reader to get in the building as there are barriers (pretty much like using your oyster card on the tube). If you forgot it the security guards wouldn't let you in even if they recognise you.
I can still remmeber a girl asking 'even if you greet the security guard everyday can they not let you through' and my tutor said no. No ID, no entry - you literally had to rush home to get it. if you lived a distance your ID is the most important thing (You can miss you lesson by the time you arrive home to pick it up). Luckily enough I always had my ID at hand, I only had one small encounter, but that was only about 2 min away from my house.
There was a guy who lived about an hour away from sixth form, whenever he realised he forgot his ID he would just go home.