Seems like you can't go a day without something in the news or on Twitter about how schools are failing and it's the fault of teachers. Regardless of outside-of-school factors it's up to teachers to cure the ills of society. Let's look at this from a strictly time matter.
There are 365.25 days in a year. Multiply that by 24 hours in a day for 8,766 hours in a year.
We have 172 days of instruction scheduled in the 2012-2013 school year. Teachers work 7.5 hours per day so let's assume 7.5 hours for kids from simplicity sake. Let's add on another 2 hours per day for a long morning and afternoon bus ride. That puts us at 9.5 hours per day for 172 days. This on the long side, but let's use this 1,634 hours as the number of hours a student would be in our care during a school year.
But wait, we also have summer school for 7 hours per day for 24 days. Let's tack on that long bus ride again to get 9 hours for 24 days. This is an additional 216 hours of contact.
1,634 hours + 216 hours is 1,850 hours a student would be in our care. Not all of these would be in direct instruction. We've included time for bus rides and the 7.5 hour day would include time for lunch, recess, class changes, transitions, etc.
At most we would have a student 21.1% of their year. That leaves 78.9% of time when a student is out of our contact. Seems to me that this nearly 80% would have some influence on student achievement.