How to make a school timetable without clashes

• The bell schedule: periods per day, their start and end times, breaks, and any shorter days.
• For each class and section, the number of periods per week for each subject.
• Who teaches each subject to each section.
• Rooms that are shared or specialised — science labs, the computer lab, the library, the sports ground.
• Each teacher's constraints: part-time days, other duties, and any periods they cannot take.

Add up the periods each teacher is assigned across all their sections and compare the total with the teaching periods in the week. A teacher assigned 38 periods in a 40-period week leaves the timetable almost no room to move; one assigned 44 cannot be timetabled at all. Fix overloads now, by reassigning sections, rather than discovering them when the grid will not close.

Do the same for rooms. If eight sections each need three lab periods a week and there is one lab with 40 periods, the lab is 60% booked before a single period is placed — which tells you how much flexibility is left.

Order matters. Place the periods with the fewest possible slots first and the most flexible ones last:

1. Double periods and practicals, which need consecutive slots and a lab.
2. Teachers who are part-time, or shared with another campus.
3. Subjects taught to split or combined groups at the same time, such as second languages and electives.
4. Each section's remaining subjects, spreading any one subject across the week rather than stacking it on one day.

Where the load allows, leave each teacher a few free periods spread through the week. They are what makes substitutions possible later without overloading anyone.

Three kinds of clash account for almost every timetable error:

On paper these are checked by scanning each column. In software they should be rejected as you make the edit. AcadiFlow's timetable checks all three while you build, and keeps each published timetable as a version so earlier schedules stay on record.

A timetable with no clashes can still be a bad one. Before publishing, look at it from the people's side:

• No section has the same subject more than twice in a day, unless it is a double period.
• Demanding subjects are spread across the week rather than all falling after lunch.
• No teacher has an unreasonable run of consecutive periods without a break.
• Teachers who move between buildings or campuses have time to get there.

When a teacher is absent it is tempting to edit the timetable. Don't. The timetable should remain the record of the planned schedule; a substitution is an override for one period on one date. Keeping the two apart means you can still say what the schedule was on any given day, and how many periods each teacher covered for absent colleagues this term.

To choose a substitute, look for teachers who are free in that period, have not already covered several periods that day, and ideally teach the same class or subject.

Timetables change during the year — a new section opens, a teacher leaves, a lab closes for repairs. Publish each change as a new version from the date it takes effect rather than overwriting the old one. Attendance records, and any dispute about which class was where, then refer to the timetable that was actually in force.

Small schools build workable timetables on paper or in a spreadsheet. The limits appear with size. Once there are several sections per class, shared labs and part-time teachers, checking clashes by hand becomes slow and error-prone, and every mid-year change risks introducing a new one. Software that validates each edit, versions each publication and records substitutions removes most of that risk. Automatic generators can produce a first draft, but the draft always needs the review in step 5.