Many people miss the bus every day, making it a viable excuse. Buses sometimes are off schedule - late and early - therefore this is a hard excuse to refute. The downside is teachers and principals around the nation have heard this excuse often. You can also cite bad traffic and many pick ups on the bus route.
Speaking of bad traffic, this tried and true excuse works if your parents drop you off or you drive yourself. If you want to take another take on the automotive excuse route, you can say that your car broke down, battery died or it wouldn't start at all. Of course, these excuses don't work if you live nearby, walk or bike to school.
The alarm clock is a good scapegoat used by many a tardy student. One excuse is the simple alarm clock didn't go off; you can expand upon that and say that you had a power outage in the middle of the night. If it is the right time, you can say that you forgot to change the time for Daylight Savings Time.
We have all left the house ill-prepared and gone back for the forgotten objects. Let your teacher know that you left your homework, books or lunch at home and realized when you were on your way to school. Being prepared sometimes trumps being a little tardy.