Assuming that you ride on this bus, the task should be quite simple.
All you have to do is time the bus ride each day. You note the time when it leaves your home and the time that it arrives at the school. Write these data down.
Then you have to decide how many days you are going to use to get your average. Continue to time the bus for that number of days. When you have timed the bus the required number of days, you take the sum of all the times you have recorded and divide them by the number of trips that you timed.
So, for example, if you time it on 10 different trips, take the total number of minutes that the trips took and divide them by 10. That will give you the average time that a bus trip takes.