i would leave them for that amount of time yes.
ds1 is 10 on thursday, and he has walked to the local shops on his own (round trip of 20 mins), and would be able to walk to school on his own, except i have to take ds2 to the same place
dd1 is 12 at xmas, and is capable of walking to and from secondary school (15 mins each way), she has a mobile phone and her own set of keys to the house in case o am late back or stuck in traffic (tho in reality atm she is being dropped because of the nasty weather, and i am out doing drop offs for the others so may as well)
when she was at the same school as ds1, they often walked to or from school in the better weather earlier this year
they are both sensible kids
i guess if they werent i wouldnt feel the same
i wouldnt leave ds2 on his own at 9 because he cant walk, so would worry he couldnt get out if there was an emergency