Unfortunately, when there are positions in schools which could be a promotion for staff, you will usually find at least one internal candidate. However they are legally obliged to open up the position to external candidates. Which tend to mean, to some extent, it is a done deal before any interviews even take place, but they have to do the interviews.
Hard to prove any discrimination, as they set the criteria for selection, which, if they include something like able to demonstrate good fit with school ethos, ability to form good relationships with the children etc, will obviously give the internal candidate an advantage.
When i was applying for teaching/education jobs, if the job involved whole days of selection, and/or long journeys to interview, i would always ask if there were internal candidates as 9/10 the internal will get it.