It might work in my opinion if they increase the number of breaks between classes. > or make lunch longer since you're going to stay for longer.
As a child my school started from 8.00 to 16.00 every day.
But we had Wednesdays off in primary.
We had lots of breaks
School was fun. Teachers didn't feel pressured by 'lack of time'
And people in the 'upper' years who had exams had more time to study and prepare.
Also on the plus side it allows parents to come out from work, (those who end work from 3-5) Quickly cook food and relax before the 'rowdy' kids come. It makes for more valuable family time.
I think the only way you can judge it is if you've lived through it. European countries live that habit/school life and they're doing significantly better than British kids.