At least you don't miss anything.
Not really. There are tons of reasons why you could be late.
And the teacher asks you why you are late. It's self explanatory, tell her that there was a lot of traffic, or whatever the reason may be.
WHY ARE YOU LATE?SIT DOWN, I DON'T WANT TO HEAR YOUR EXCUSES.:/
It's not self-explanatory, though. If there are multiple possible reasons, how can you know which it was?