You would be at school for 7 hours and 10 minutes.
EXPLANATION:
7:20 to 12:00 is 4 hours and 40 minutes.
12:00 to 2:30 is 2 hours and 30 minutes.
Then add together the 4 and 2 to get 6 hours. Then add the 40 and 30 to get 70 minutes. Afterwards you convert the 70 minutes into hours which would be 1 hour and 10 minutes. Then add 6 hours to that to get an answer of 7 hours and 10 minutes.
