Guide
Hard rules vs soft preferences in school timetables
Hard rules are requirements the timetable must respect, such as teacher unavailability, room closures, or impossible overlaps. Soft preferences are goals that improve quality when possible, such as spreading lessons across the week. Use hard rules sparingly so generation keeps enough flexibility.
Last updated: April 28, 2026 ยท Reviewed by the Schedull product team
Rule types at a glance
| Rule type | Best use |
|---|---|
| Hard rule | A mandatory condition, such as a teacher being unavailable, a room being closed, or two sessions that cannot overlap. |
| Soft preference | A desired pattern, such as morning lessons, balanced subject spacing, or avoiding late slots when possible. |
| Manual edit | A final human adjustment after generation when the coordinator wants to tune a specific result. |
Examples
Good hard rules
- A teacher is unavailable on Tuesday morning.
- A lab is closed during a specific module.
- A teacher cannot be in two classes at the same time.
- A fixed session must happen in one known slot.
Better soft preferences
- Spread a subject across several days.
- Prefer a subject in the morning.
- Avoid the last module when another good slot exists.
- Keep teacher workload balanced through the week.
Hard rule, preference, or manual edit?
Use this quick test before adding a rule.
| Question | Use |
|---|---|
| Would the timetable be unacceptable if this condition is broken? | Use a hard rule. |
| Would the timetable still work, but be less convenient? | Use a soft preference. |
| Is this a one-off judgement after seeing a result? | Use manual editing. |
| Are several hard rules competing for the same few slots? | Loosen preferences or add more possible slots. |
How to choose the right rule type
Mark only true blockers as hard
If the school cannot accept a placement under any circumstances, make it a hard rule.
Use preferences for ideal patterns
If the timetable is still usable when the pattern is missed, make it a preference.
Review no-result cases
When generation cannot find a timetable, look for hard rules that compete for the same limited slots.
Refine after generation
Use manual editing for small human choices instead of turning every preference into a mandatory rule.
Why too many hard rules block generation
Every hard rule removes possible placements. That is useful for real impossibilities, but dangerous for preferences. If a class has many subjects, a teacher has limited availability, and a room is also restricted, a few unnecessary hard rules can leave no valid combination.
Related guides
Use these pages when you need the full workflow or teacher availability guidance.
FAQ
Why not make every rule mandatory?
Too many mandatory rules, or hard constraints, can leave no valid placement for classes, teachers, or rooms. Use them for real blockers such as teacher unavailability, closed rooms, fixed sessions, or lessons that must not overlap. Put quality goals into soft constraints so generation can still find a workable timetable.
Can soft preferences still matter?
Yes. Soft preferences and soft constraints guide the timetable toward better patterns without blocking a valid result. They are useful for spreading lessons, reducing late periods, or improving teacher workload when the school has enough room to choose between options.
What should be a hard rule?
Use hard rules for real impossibilities: unavailable teachers, closed rooms, fixed sessions, or lessons that must not overlap. If a rule describes a preference, such as a better time of day or a nicer distribution, keep it flexible. If it only matters after seeing a generated timetable, handle it with manual editing.