Once DP subject choices are collected, the next question is whether they fit into a timetable. Most schools use option blocks: a fixed number of parallel time slots, each containing several classes, with every student taking one class per block. Getting the blocks right decides how many students get the combination they chose. This article explains how to move from a list of choices to a workable block grid, how to spot clashes and what levers you have when the first draft does not work.
What option blocks are
An option block is a set of classes taught at the same time. If your grid has six blocks, each student takes one class in each block, and the six classes make up their programme. Two subjects in the same block cannot both be taken by one student; that is a clash.
A simple example with four blocks:
| Block A | Block B | Block C | Block D |
|---|---|---|---|
| Biology HL | Chemistry HL | Physics SL | History HL |
| Economics HL | Visual arts SL | Psychology SL | Biology SL |
| English A HL | Mathematics AA HL | Mathematics AI SL | Spanish B SL |
A student who chose Biology HL and Economics HL clashes in Block A. A student who chose Biology HL, Chemistry HL and Physics SL fits, one per block.
Real grids for the DP usually have more blocks and more classes per block, but the logic is the same.
Start from clean data
Block planning only works with valid, final choices. Before you start:
- make sure every student has a complete, validated choice (see validating DP subject combinations);
- decide which subject levels below their minimum you will still consider (see minimum group sizes for DP subjects);
- know your constraints: number of blocks available, maximum class size, rooms and teacher availability.
Step 1: count the pairs
The basis of block planning is knowing which subject levels are chosen together. For each pair of subject levels, count how many students chose both. Pairs with many shared students must not sit in the same block. Pairs with no shared students can safely share a block.
You can do this in a spreadsheet with a co-occurrence table, but with more than a few dozen students and twenty-plus subject levels it quickly becomes hard to read.
Step 2: draft the first grid
Place the largest subject levels first, spreading those that share many students across different blocks. Then fill in smaller subject levels where they create the fewest clashes.
For each draft, look at three numbers:
- Students with a clash-free timetable. The main measure of a good grid.
- Students with at least one clash, and which subjects cause it.
- Clashing pairs per block, so you can see which placements to change.
Step 3: use the levers
When the first draft leaves students with clashes, you have several options.
Offer a subject in two blocks
A popular subject level can run as two parallel classes in different blocks. Students then take it in whichever block suits their other choices. This costs an extra class, so it works best for subjects that are large enough to justify two groups.
Merge SL and HL into one class
Some schools teach SL and HL students of the same subject together for shared content, with HL students having additional lessons. In the block grid, merging means one class covers both levels, which frees a slot. Whether this is appropriate depends on the subject and your teaching arrangements; check the current IB subject guides and agree it with the subject teacher.
Respect maximum class size
Every class has an upper limit, for teaching reasons and because of room sizes. A grid that puts thirty students into one class is not a solution. If a subject level exceeds the maximum class size, it needs a second section anyway, which also gives you scheduling flexibility.
Include or exclude marginal subjects
Subject levels below their minimum complicate the grid. Try one draft with them and one without: the difference shows how much keeping a small class costs in clashes for everyone else.
Change the number of blocks
More blocks reduce clashes but use more of the timetable. If you have room for an extra block, test whether it significantly improves the result before committing.
Step 4: compare scenarios
Rarely is there one obvious answer. Save several scenarios and compare them side by side:
| Scenario | Blocks | Max class size | Students clash-free | Notes |
|---|---|---|---|---|
| A | 6 | 20 | to be measured | All subject levels included |
| B | 6 | 20 | to be measured | Below-minimum subjects excluded |
| C | 7 | 20 | to be measured | Biology HL in two blocks |
Fill in the numbers from your own data and discuss the trade-offs with leadership: a seventh block, an extra class or a closed subject level each have a cost.
Step 5: resolve the remaining clashes
Even the best grid can leave a few students with clashes. Options include:
- asking the student to change one subject, in a corrections round limited to viable subject levels;
- moving a subject to a different block if that does not create new clashes;
- arranging an exceptional solution for one student, documented as such.
Talk to each affected student individually. Knowing exactly which pair clashes, and in which block, makes that conversation concrete.
Handing over to the timetabler
The block grid is a draft for the timetabler, not the final timetable. Hand over the final choices as CSV, the chosen scenario with its blocks and the list of any remaining clashes. The timetabler then adds teachers, rooms and periods in their own software.
Checklist
- All choices complete and validated before planning starts.
- Decision taken on subject levels below their minimum.
- Number of blocks and maximum class size agreed.
- Pair counts reviewed for the largest subject levels.
- First draft measured: students clash-free, students with clashes, clashing pairs.
- Levers tested: parallel sections, merged SL and HL, extra block.
- Several scenarios saved and compared with leadership.
- Remaining clashes resolved with individual students.
- Final choices and chosen grid handed to the timetabler.
Frequently asked questions
How many option blocks does a DP timetable need?
At least as many as the subjects each student takes, so six for a standard programme, and often more to reduce clashes. The right number depends on your subject offer and timetable capacity.
Is it acceptable to teach SL and HL in one class?
Many schools do it for shared content, with extra lessons for HL. Whether it suits a particular subject depends on your arrangements; check the current IB subject guides and discuss it with the teacher.
Can block planning be automated?
Software can propose a layout that maximises clash-free students, but the result still needs judgement: costs, staffing and priorities are decisions for the school.
See MyIB with your own subjects
MyIB is independent IB subject selection software for schools running the IB Diploma Programme. Its what-if block simulator lets you set the number of blocks (up to 12) and maximum class size, place subject levels, offer a subject in several blocks, merge SL and HL, include or exclude below-minimum subjects, and see clashes per student and per block. An automatic solver proposes a layout and you can save and compare scenarios. Data goes to your timetabling software via CSV export. Book a demo or check pricing. For the earlier stages, see how to run DP subject choices.