Don't worry everyone has the same thoughts around stressful times, like exams. I misplaced my loan form yesterday and thought the world was going to end, i foudn it this aftrenoon though and all is fine.
I check for pens and exam entry forms and other bits all the time when im on my way and get others to check too, just incase im halucinating maybe? hehe!
Its normal to worry, just sit down and have a cup of tea and relax, it'll all work out in the end, I however when i'm stressed will not listen to anyone tell me this and will honestly think everything is just a mess, but dont let yourself get like that, take some deep breaths and think of all the great things you'll be able to do when exams have finished and how nice it'll be not to have to look at a textbook again!
My exams start next week, so im calm at the moment but by next week i'll be bouncing off the walls, but its only a few hours of your time, is hardly worth it when you think of the actually time of the exam and how much more time you spend worrying about the damn thing!
As for teachers not entering you, its best to check, i wasnt entered for any exams last year the useless college admin, so it best to check because there all a bunch of eejits
Good luck, if you still have exams left!