A two-week timetable exists because one week is not always enough to say what a school teaches. Some subjects need an hour and a half a week. Some rooms can only be shared fairly over a fortnight. Some teachers are split between more classes than a single week can hold.
It is also a promise, repeated for every alternating lesson, that thirty students and a teacher will know which week it is. This article is about when that promise is worth making, how to model it, and where it usually breaks.
What an A/B timetable actually is
Two weeks with the same days and the same periods. Most lessons appear in both. A few appear only in week A, a few only in week B, and the two weeks alternate for the rest of the year. (A rotating day cycle, where "day 1" to "day 6" ignore the calendar week, is a different structure.)
When a school genuinely needs one
Odd weekly hours
The clearest case. If the curriculum gives a subject 1.5 hours a week and your periods are whole units, there is no honest single-week answer. Rounding up steals time from another subject; rounding down under-delivers the curriculum. One period every week plus one extra period every other week delivers exactly three periods a fortnight.
Fortnightly lab, sport or computer-room slots
A school with one well-equipped lab and six classes that each need a double practical does not need six doubles every week — it may only have room for three. Three classes in week A, three in week B, and every class gets its practical every fortnight. The same logic applies to a pool booked from outside, or a specialist room with one technician.
Shared specialist teachers
A music or computing teacher with a reduced contract, or one who also teaches at a partner site, may have fewer free periods than the classes they serve. Alternating lets one teacher cover class 7a on Tuesday period 3 in week A and class 7b in the same period in week B.
When it is a workaround
A/B weeks have a running cost: everyone now reads two plans instead of one. That is worth paying for a structural reason, rarely to avoid a small one.
- Fixing one clash. A single teacher conflict is not a reason to put the whole school on a fortnightly cycle. Change an availability, move a lesson card, or accept one weaker slot.
- Hiding an over-full grid. If a class's hours do not fit into a week, alternating a subject makes the plan fit on paper — and quietly halves that subject. That is a curriculum decision, and it should be taken as one.
- Inheriting last year's structure. Schools often carry A/B weeks forward because the plan they started from had them. Ask what each alternating lesson is for. If nobody knows, it is probably a leftover.
A useful count: how many lesson cards actually alternate? If the answer is four, ask whether four lessons justify a second timetable for everyone.
How to model it in Bildena
Two settings, in two places. The school's week cycle is either a single week or A/B week (two-week cycle). Each lesson card then carries a week pattern: every week, odd weeks or even weeks. Odd weeks is week A; even weeks is week B. Cards that do not run every week show the pattern as a badge, so they are easy to find in a long list.
The hours on a card are the hours in the week the card runs. The 1.5-hour subject from above becomes two cards:
| Card | Week pattern | Week A | Week B |
|---|---|---|---|
| Chemistry 9b | Every week | 1 | 1 |
| Chemistry 9b (extra) | Odd weeks | 1 | — |
| Fortnight | 3 periods, 1.5 a week on average | ||
A fortnightly practical is a single card on odd or even weeks with a double length. Blocks behave as they do in any other week; our article on doubles and blocks covers how they are spread.
If you are coming from aSc TimeTables, a two-week file arrives already set to an A/B cycle, with each card's week pattern carried across.
The pre-flight check catches two modelling mistakes early. Cards marked odd or even weeks in a school still set to a single week are flagged as critical, because without a second week those patterns have nothing to alternate against. And a family of parallel cards that mixes week patterns gets a suggestion: the cards are locked to the same slot, but in practice they would run in different weeks.
What the solver does with two weeks
Our solver, tuned specifically for distributing lessons across a school week, treats an A/B school as two weeks that share one grid.
- Clashes are checked week by week. A teacher, a class, a group, a room and — where you schedule by student choice — an individual student can each be used once per period in each week. A week-A chemistry card and a week-B physics card can share Tuesday period 3, the same lab and the same teacher. Two week-B cards in that period cannot, however empty it is in week A.
- Teacher gaps are counted per week. A teacher with a lesson in period 1 in week A and in period 3 in week B has no gap in either week. Laying the two weeks on top of each other would invent one. A limit on gaps per week applies to each week separately.
- Some limits are counted across both weeks. A teacher's maximum lessons per day counts a week-A and a week-B lesson in the same period as two. That errs on the safe side, but a very tight daily maximum may leave a plan less full than it could be.
The same caution applies to the class-hours total in the pre-flight check: it currently adds week-A and week-B hours together, as if both ran every week. For most classes that is harmless headroom. A class planned right up to the edge of the grid may see a warning that each individual week would pass.
The three pitfalls
1. People confusing the weeks
The biggest risk has nothing to do with the timetable itself. Decide, in writing, what week A means. Odd calendar weeks? The first week of each term? After a holiday of odd length, those two definitions drift apart, and half the staff will follow one while half follow the other.
Then publish the answer: a list of the A and B dates for the whole year, and the week letter on every printed or shared plan. A grid that shows two lessons in one period without saying which week each belongs to is the fastest way to get a class standing outside the wrong room.
2. Clashes that only exist in week B
A generated plan is checked in both weeks. The danger comes afterwards, when someone moves a lesson by hand while looking at week A. The room is free, the teacher is free — in week A. Before publishing any manual change in an A/B school, look at the other week too.
3. Uneven load between weeks
Each card's hours are fixed, so the load of each week is decided when you assign week patterns — not when you generate. The solver cannot move an hour from week A to week B.
An example: class 8b has 28 periods every week, three odd-week cards and one even-week card. Week A holds 31 periods, week B holds 29. A teacher whose alternating cards all sit in odd weeks has a heavy week and a light one, every fortnight, for a year.
The fix is on the data side. Pair alternating cards so that, for each class and teacher, what runs in week A is roughly matched in week B. Total the hours per week before generating; it saves a conversation in October.
The honest summary
A/B weeks are the right tool when the curriculum, the rooms or the staffing genuinely do not fit into one week. They are the wrong tool for making a hard week look easy. The solver will handle two weeks correctly; the harder part is making sure the people reading them can too.
In Bildena the week cycle is a single school setting and each lesson card carries its own week pattern, so trying alternating weeks on a handful of lessons is a few minutes' work, not a rebuild.