The School Day has changed a number of times over the years. Here is a collection of timetables we've built up so far:
Boys'
School, 1938
Boys'
School, 1939/40
Turn
of the '80s
Turn
of the '90s
1992(?)-2003
2003+
And here's a comparative tabular view of the timetables we've got:

Notice that the day has gradually shunted earlier, and that the lunch break has halved since the '40s.
Here's a chart showing the balance between lesson time and recreational time (including dinner-break):

Worktime has increase by 30' and recreation has decreased by 40' (20' if you take into account the reduction in the length of the school day) since the turn of the '40s.
In the early weeks of the Girls' School, afternoon timetables were regularly suspended to make way for school debates, including such motions as "It is wrong to pretend there is a Father Christmas", or "A Caravan is a more desirable place of residence than a house". In both these cases, the motion was lost.
The Boys' timetable in 1938 (devised by Mr Wreakes) looked like this:
9:00am
Register
marked
9:02am
Religious
Instruction
9:35am
Period
1 (40')
10:15am
Period
2 (35')
10:50am
Break
(10')
11:00am
Period
3 (30')
11:30am
Period
4 (30')
12noon
Dinner
1:30pm
Register
marked
1:32pm
Period
5 (33')
2:05pm
Period
6 (30')
2:35pm
Break
(10')
2:45pm
Period
7 (30')
3:15pm
Period
8 (45')
4:00pm
Hometime.
Worktime: 4:33'
Recreation: 1:50'
The Boys' timetable in 1939/40 looked like this:
9:00am
Register
marked:
The
register closed at 9:45.
9:02am
Religious
Instruction
9:35am
Period
1
10:20am
Break
10:30am
Period
2
11:15am
Period
3
12noon
Dinnertime
1:30pm
Register
marked:
The
register closed at 1:45.
1:32pm
Period
4
2:17pm
Break
2:30pm
Period
5
3:15pm
Period
6
4:00pm
Hometime
Worktime:
4½
hours
Recreation: 1:53'
In November 1942, the day was shortened to take into account the blackout. It started at 9:30 and finished at 3:45. The following year's blackout timetable may even have finished at noon.
The school day was longer for High School pupils in 1963/4 than it had been at the Secondary Modern the year before. In 1967 it changed again, running from 8:40-12:00 in the morning and 1:20-4:00 in the afternoon. This new school day, overseen by Mr J E Wilkinson, was the first at the High School to include double periods.
Martin Brown describes the regime in the early '80s:
8:50am
Registration:
25
mins as this allowed for assemblies (on your year's allotted day) and
when
it wasn't you were supposed to do some social/personal development with
your form tutor - but I think this was at their discretion and depended
largely on what marking / lesson plans they had to do
9:15am
Five
mins to get to your first morning lesson
9:20am
First
lesson
10:30am
First
break
10:50am
Second
lesson
12noon
Dinnertime
1:10pm
Afternoon
registration
1:15pm
Five
mins to get to your first afternoon lesson
1:20pm
First
afternoon lesson
2:30pm
Five
mins to get to your second afternoon lesson
2:35pm
Final
lesson of day
3:45pm
"home
sweet home"
Worktime: 4:40'
Recreation: 1:30'
The lessons above
are shown as double lessons, but single lessons of 35' also existed.
"If
you had a single lesson the teacher would have to let you go so you
could
get to your next lesson in time. This wasn't too bad if you were in
somewhere
in school close by (esp. if in neighbouring classes as teachers would
swap
and not pupils), but a real pain if you were over the other side of
school!!!"
8:45am
Registration
/ Assembly:
Assemblies
were run by year. Monday for 5th year, Tuesday for 4th etc. (6th Form
had
an assembly in Lower School on a Friday, if I recall correctly). Taking
a register in such circumstances was largely farcical which was a bonus
for those who wanted to skip the assembly.
9:10am
Period
1
10:20am
First
Break:
Usually
included the obligatory visit to the Osborne/Hatfield kitchens for a
flapjack
or something similar.
10:40am
Period
2
11:50am
Dinnertime:
6th
Form and Osborne did meals. Sandwiches elsewhere. Massey's for
potato-shaped
grease, or the Fish Net for chips. Or maybe a sandwich from further up
the street (Armstrong's butchers etc). All followed by football on the
coach park or whatever.
12:35pm
Registration:
A
rushed affair, often narrowly missed.
12:40pm
Period
3:
The
afternoon lessons were five minutes shorter.
1:45pm
Last
Break
1:55pm
Period
4
3:00pm
Freedom.
This schedule was reappraised a couple of years later, in an attempt to fit in the national curriculum. Despite the fact that I had to live by it for about five years, I can hardly remember it. At a guess it was:
8:40am?
Registration
/ Assembly
(after
the demise of Lower School, 6th Form took over the Friday slot in the
Main
Hall, with the 1st year presumably getting shoved in with the second)
9:00am
Period
1
10:00am
Movement:
A
cunningly devised break designed for teachers to have a fag in.
10:05am
Period
2
11:05am
First
Break
11:15am
Period
3
12:15pm
Dinnertime
1:00pm?
Registration
1:05pm
Period
4
2:05pm
Last
Break
2:15pm
Period
5
3:15pm
Freedom
My times might not be quite right there. But presuming the lengths are right, worktime increased from 4½hrs a day to 5hrs a day, while recreational time increased by five minutes to 1hr 10mins. These are in pretty much the same ratio, so it wasn't so bad. Except the school day was at least 15min longer, eating into out-of-school leisure time, and that's not even considering a possible increased homework load.
As of September 2003, the timetable has gone all bipartite, with the introduction of a weekly PSE day. The day of the PSE day varies from term to term.
NORMAL DAY:
8.35am
Registration
/ Assembly
(Y11
Monday - Y7 Friday, No 6F Assemblies)
8.50am
Period
1
9.50am
Movement
(or fag brake)
9.55am
Period
2
10.55am
Break
11.15am
Period
3
12.15pm
Lunch
1.05pm
Registration
1.10pm
Period
4
2.10pm
Movement
2.15pm
Period
5
3.15pm
HOME
SWEET HOME
(That doesn't seem a hell of a lot different to the orangey timetable above. But it isn't EXACTLY the same because afternoon break has now been annihilated.)
PSE DAY
8.35am
Registration
/ Assembly
8.50am
Period
1 (5' shorter than normal)
9.45am
Movement
9.50am
Period
2 (5' shorter than usual)
10.45am
Break
(5' shorter than usual)
11.00am
Period
3 (5' shorter than usual)
11.55am
Movement
12.00
Noon
PSE
Tutor Period (PSE lesson taught by form tutors)
12.25pm
Lunch
1.15pm
Registration
1.20pm
Period
4 (5' shorter than usual)
2.15pm
Movement
2.20pm
Period
5 (5' shorter than usual)
3.15pm
HOME
Quite clever bit
of fiddling that (albeit somewhat evil)...
By 2011, the
timetable has been tweaked ever so slightly, with extra time at morning
registration, and with afternoon registration abandoned as unnecessary
now that pupils aren't allowed out of the cage at lunchtimes. The day
now looks like this:
8.35am
Form Tutor Time / Assembly
8.55am
Lesson 1
9.55am
Change over
10.00am
Lesson 2
11.00am
Break
11.20am
Lesson 3
12.20pm
Lunch
1.05pm
Students move directly to Lesson 4
1.10pm
Period
4
2.10pm
Change over
2.15pm
Lesson 5
3.15pm
End of school