Constraint recipe
A lab block is a double-period practical session in a specialist room. Both periods need the same teacher, class, lab, and equipment, so the block reserves a scarce room for two periods at once. Place lab blocks before flexible lessons, and when a clash appears, check the whole block rather than moving one period of it.
Key takeaways
- A lab block needs two adjacent periods in the same lab.
- The block reserves the lab for both periods; every other class waits.
- Capacity can make a free lab unusable for the group size.
- Place scarce lab blocks before flexible classroom lessons.
One lab, many claims
Labs are scarce and shared across year groups, and school timetabling is hard enough that placement order matters: the items with the fewest valid slots go first. A lab block is usually the most constrained thing in the week, because it stacks a room requirement, an adjacency requirement, and a teacher onto one placement.
Treat lab blocks as part of data preparation, not as a post-generation edit. A block added after the timetable exists usually forces a rebuild.
A worked example: one chemistry lab, one week
This is the chemistry lab's week, not a teacher's. 9A holds a double block on Tuesday P3 and P4. A proposed move of 8B's single lesson into Tuesday P4 collides with the second half of that block, which is easy to miss when you only look at where a lesson starts.
Lab occupancy: the block holds both periods
| Mon | Tue | Wed | Thu | Fri | |
|---|---|---|---|---|---|
| P1 | 10B | ||||
| P2 | 8B | ||||
| P3 | 9A 1/2 | 10B | |||
| P4 | 9A 2/2 · 8B? | ||||
| P5 | 7C | ||||
| P6 |
- Single lab lesson
- 9A's double block
- Attempted placement into the block
Before moving anything, open the whole block: teacher availability, class availability, lab availability, equipment, and whether the group even fits the lab. Splitting the block into two singles is only an option when the curriculum allows it; a practical that needs sustained time is a block for a reason. For workshops and vocational labs, see workshop and lab scheduling.
Place the blocks while options exist
- List the lab sessions that require adjacent periods, and map each to its eligible labs and required equipment.
- Compare each group's size with the lab's capacity before treating the lab as eligible.
- Place the most constrained lab blocks first, before flexible classroom lessons take the good slots.
- When a clash appears, check both periods of the block and the equipment, not just the first period.
- Split a block into singles only when the curriculum explicitly allows separate sessions.
Lab-block mistakes
- Checking the room type but not whether the lab fits the group size
- Scheduling two blocks that depend on the same limited equipment at once
- Splitting a block that exists because the activity needs sustained time
In Smootables
A lab block combines two settings on one placement: a longer lesson duration for the block, and Allowed rooms restricted to the eligible labs. The generator places the session as one continuous stretch in one of those rooms, and the Room no-conflict rule keeps the lab unique for its whole length, so the second half of a block can never be double-booked the way the worked example above shows.
Lessons marked Remote (no room) do not hold a physical room at all, which keeps online theory sessions from competing with practicals for lab space.
- Set the block's duration and its Allowed rooms before generating, not after.
- Mark genuinely online sessions as Remote (no room) so they stop consuming lab capacity.
Quick answers
Are lab blocks mandatory rules?
They are mandatory when the activity cannot run without the lab or the adjacent periods. Otherwise they can be a preference.
Why do lab blocks cause room clashes?
A block holds a scarce room across two periods, so one placement collides with every flexible lesson that wanted either period.