I'm doing 14U, but our school allots 13U in the timetable, so there's no formal Extension 2 English "class" or "period".
This is how we've structured it at my school:
Line 1 - English
Line 2 - Maths
Line 3 - Studies of Religion (1U)
or Catholic Studies
Line 4 - Elective
Line 5 - Elective
Line 6 - Elective
Line 7 - Elective
So on a given day, you would have periods which correspond to the given line. So you'd have double English, maths, religion, etc.
With Line 7, however, it is further subdivided into 7A and 7B.
7A = Extension 1 Maths
7B = Extension 1 English
Obviously, if you do 7A and 7B, you won't have another "Elective" (for example Biology), you'd have your timetable split up into 3U Maths lessons and 3U English lessons (when other people are doing their Line 7 Electives).
It works well, unless you do 4U Maths or 4U English.
For 4U Maths, you'd have morning classes (I would imagine) as well as classes on line 7B. That makes you wonder, how can a 4U Maths person do 3U English? Well, simply put, at my school, nobody does 4U maths
and 4U english so the periods are fine, LOL.
If you do 4U Maths:
7A: 3U maths
7B: 4U maths
If you do 4U English:
7A: 3U maths
7B: 3U english
It's not that complicated, because most of the time you'll have people doing either 4U Maths with just the 2 units of english, and at other times you'll have people doing the 3/4U of english, who are only doing 2U maths. (I hope I havent confused you all).
You can always drop subjects, so therefore you'll have study periods where you'd normally have the periods (which have been dropped)
My 7A = 3U Maths,
my 7B = 3U english.
If I dropped 3U maths, it'd be a study period, or, I can turn it into a 4U english "class" per se
picking up 4U english isn't a problem, because there are no classes/lessons!
Phew, I hope that makes sense, because it works well at my school.