Emily leaves her house 30 sec after 8:25 AM to bike to her school,which is 4.25 miles away. At exactly 8:30 am Emily passes the post office. Which is 3/4 miles away from her house. Emily must get to school on later than 8:51 am to be on time to her first class at 9:00 am. What time will be be there
Emily leaves her house at 8:25 AM + 30 seconds = 8:25 AM + 0.5 minutes = 8:25 AM + 0:30 minutes = 8:25 AM + 0:30 minutes after 8:25 AM = 8:25 AM + 8:25 AM = 8:25 AM + 8:30 AM = 8:55 AM.
This means that it takes Emily 8:55 AM - 8:30 AM = 25 minutes to bike from the post office to her school.
Therefore, Emily will arrive at her school at 8:55 AM + 25 minutes = 8:55 AM + 0:25 minutes = 8:55 AM + 0:25 minutes after 8:55 AM = 8:55 AM + 8:55 AM = 8:55 AM + 9:00 AM = 9:20 AM.
Since Emily has to be at her school by 8:51 AM, she will be late by 9:20 AM - 8:51 AM = 0:29 minutes.
This means Emily will be late by 29 minutes.
Thus, Emily will be at her school at 9:20 AM + 29 minutes = 9:20 AM + 0:29 minutes = 9:20 AM + 0:29 minutes after 9:20 AM = 9:20 AM + 9:20 AM = 9:20 AM + 9:30 AM = 9:50 AM.
Thus, Emily will be at her school at 9:50 AM. Answer: \boxed{9:50 \text{ AM}}.