A timetable clash is not one problem. It is six, and they fail in different ways. The first is obvious enough that everyone checks for it. The other five are the reason clashes still reach publication in schools that check carefully.
The first is the teacher clash: one member of staff timetabled to teach two classes in the same period. This is the clash everyone knows about, and it is the easiest to spot because two groups of students physically arrive at two rooms expecting the same person. It is also the easiest to prevent, which is why it is rarely the one that causes the crisis.
The second is the class clash: one group timetabled for two lessons at once. This happens more than people expect in schools with option blocks and setting, where a student sits in more than one grouping and the groupings are timetabled independently. The clash is invisible at the class level and only appears at the level of the individual student, which is exactly why it survives a review of the master grid.
The third is the room clash: two lessons assigned to the same room. This one tends to be caught late because rooms are often assigned after the lessons are placed, as a separate pass — and a second pass over an already-full grid has very little room to manoeuvre.
The fourth is the qualification clash, and it is the first of the ones that never announce themselves. A lesson is assigned to a teacher who is not qualified or approved to teach that subject. Nothing collides, no two people are in the same place, the grid looks perfect. It surfaces in week two of term when someone realises the Year 10 Chemistry group is being taught by a Biology specialist because the grid needed a body in that slot.
The fifth is the availability clash: a lesson placed in a period when the teacher is contractually not there. Part-time staff, protected non-contact time, staff shared with another campus, a colleague who does not work Fridays. Again nothing visibly collides. The clash only appears when the person does not turn up, because they were never supposed to be there.
The sixth is the capacity clash: a class of 32 assigned to a room that seats 24, or a practical subject assigned to a room with no equipment. Technically schedulable, practically not teachable, and usually discovered by the teacher on the first day.
The pattern is worth naming. The three clashes schools reliably catch are the ones where two things visibly collide. The three they miss are the ones where a single assignment is quietly invalid on its own. Human review is good at spotting collisions and poor at spotting invalid singletons, because checking a singleton means cross-referencing the grid against a separate list — qualifications, contracts, room capacities — for every one of several hundred lessons.
Which is why the prevention strategy that actually works is not more careful review. It is refusing to create the clash in the first place.
That means the qualification list, the availability list and the room capacities are not reference documents consulted during review — they are constraints checked at the moment of placement, so an invalid assignment is never made. A teacher who cannot teach Chemistry is not a candidate for a Chemistry lesson. A period when someone is unavailable is not a slot. A room that seats 24 does not appear as an option for a class of 32. The invalid arrangement is not created and then caught; it is never created.
The second half of prevention is re-checking the finished timetable mechanically, against all six clash types, and reporting the count. Not because a well-built timetable should contain any, but because "zero clashes, verified" is a statement you can act on, and "it looks right to me" is not.
The third half — worth stating because it is where most published clashes actually originate — is that every mid-year change must go through the same checks as the original build. Most clashes that reach staff do not come from the July build, which was reviewed carefully. They come from the March amendment that someone made directly on the grid because it was only one lesson.
Skoolia’s scheduling treats all six as hard constraints during generation rather than as a review checklist afterwards, re-validates the finished timetable, and applies the same checks to every subsequent change. If you want to watch the singleton constraints work, the free generator on this site will not let you assign a teacher to a subject they do not teach, or schedule anyone on their day off — the option simply is not offered.