We definately don't need to be going down the route of line arrays, delays or anything of the sort. We just need to get the headmaster's voice to the back of the school hall while remaining intelligible. A secondary requirement may be the occasional need to stream music from a laptop through it. That's it. Doesn't need to be pro, pimped out or mega powerful. It just needs to be some small sturdy unit with a pair of speakers that will project enough (I'm sorry I still don't have the dimensions of the hall) (or indeed the budget!).
The hall a largish, but not that large, echoey space, but not like a basketball court or anything.
OK. So, depending on HOW echoey, maybe a simple pair of cabinets at the front will give voice clarity at the back, maybe you DO need to look at distributed speakers with delays or line arrays. There's no point in sticking your heels in and deciding a pair of speakers WILL do the job.