I doubt if the majority of those who enter your race would expect chip timing - I imagine that they enter to support the school, therefore chip timing is not so important to them.
If you provide chip timing, then you'd have to make sure that you provide all the other facilities that go along with that kind of a race - maybe you already do, but if not, you might find you attract more people with the chip timing, but equally, they expect more from you.
Our village has been running a 10k for the last few years. Initial timing was two people standing at the end of the race, each with a stop watch, pad and pencil, taking it in turns to record the timing.
I'm not sure that it's changed much, but entries pretty much double each year and no-one complains!