LazyLouLou - I absolutely agree that it is no-one's business, if a teacher stays up late drinking and dancing - I assume that, in this instance, it wasn't affecting how the teacher did her job. It would be different if a teacher was staying out late, getting drunk every night, and the subsequent hangovers and fatigue were affecting how they did their job - but I'd expect to get short shrift from a head teacher if I complained that Miss X or Mr Y was out late, partying on one occasion.
I can see the justification for contacting the head teacher in the example the OP gives - the police might not be able to do anything unless they witnessed the behaviour, whereas the head could say, 'Look - X alleges they have seen you on your phone whilst driving - if this is true, can I suggest that, as it is dangerous behaviour, you shouldn't do it again'.
And given how dangerous it is, to use your phone/text whilst driving, I wish there was something I could do, when I witness it - the same goes for all dangerous driving.
Drivers must be aware that they are in control of a deadly weapon. A lapse in concentration could be all it takes for them to mow down a cyclist, a motorcyclist or a pedestrian, or to drift out of their lane, causing another driver to have to swerve - maybe causing a multiple pile up. Which is worse - shopping someone to their employer/boss/the police, or them going on to kill or seriously injure someone?
We live in a world where mobile phones have become a vital part of every day life, and where cars have become much easier to drive - all of which has combined to make some people think it is OK - and safe enough - for them to use their phones whilst driving. But we also live in a world where cars are faster, and where there is so much more traffic, which means that driving requires a lot of concentration - and using your phone/texting reduces that concentration dangerously.