@ZeroHour and @SimpleSi
From experience it is not the key evidence but is darned helpful to find the relevant information, cutting hours / days / weeks from investigations.
Of course ... if you are sure that every single device which can connect to your network will always be logged and tracked then that is ok ... and I am sure that all networks around the world are set up that way, especially so in schools which are renowned for having a lot of investment in time and money.
As for MAC address spoofing ... yep, and the concerted script kiddie will look at this as an option ... but applying reservations will mean they have to snort a MAC address, identify the machine owner / user / location, ensure that it is not on / connected when they try to connect and that should the device be on then that the accompanying error message will not get dealt with. If they do get on (and this sometimes happens in schools on guest mobile networks ... apparently) then the logs of the APs can show you the approx area where the connection took place, etc ... or if this is not possible (though with all the investment in managed wireless networks which has happened in every single school it would be surprising) then you have at least put a little something in place to be a tiny bit disruptive to the annoying brat trying to get onto your network.
For some it might slow them down by a few hours ... for others it confuses them too much and they drop it.