The Old Man and the Students Quiz

Briquinex invites you to visit the park. While enjoying a cake and an apple juice you are given a mental quiz.

You are told that : In a park an old man meets some students and asks them : What class are you? They answer him: we are all in the first class except 12, we are all in the second class except 12, we are all in the third class except 12, we are all in the fourth class except 12. Briquinex would like for you to find out how many students are in total.

photo source: wikipedia