Is this even fair at all?

Ok so today i was going to my c block. and i came in like right after the bell rang. my teacher was like u owe me 20 mins. i was like um why?? and she said i need to come on time and that im never late but i owe her 20 mins. and this was my first time being late. i also came into the class like 2 weeks or so later than everyone else so i was never aware of that rule. i tried explaining to her i cant stay after school cuz my mom gets me on her break ( i go to school in a diffrent city) and shes like well tell her to come 20 mins later. i tried telling her its not that simple. i mean i carpool also and they need to be home for a certain time. i told all my friends and they thought that was the stupidest reason ever. i mean i can see if you come in like 2 mins late or something. but comming in a few seconds late is natural just cuz u have to try to dodge and get around students. tell me if you think this is fair at all.