The one-off advert is designed to find out whether a particular approach works.
However, simply doing just one email might not tell you all you need to know. For example, if the results are poor, it will be hard to know if this was because:
You emailed the wrong person in the school
The way the advert was written wasn’t right
The headline or subject line didn’t work particularly well
You emailed at the wrong time - for example, just after a competitor has come in with a better offer
Your email grabbed attention and people took the next step (for example going to your website) but then stopped there.
The traditional response to this difficulty is to do a split test (often known as A/B testing) but this technique (derived from postal marketing) only allows the testing of one item against another (typically one headline vs another).
A/B split testing is a good idea if you have already created an advert that you know normally works and simply want to improve it a little, but it doesn’t help too much if you have not tried creating adverts for this product before or in this style before.
In such a case it is worth building testing into a series of emails which start from approaches known to work (rather than approaches that one has seen others using) and then start varying the tactic.
But perhaps the biggest lesson to learn is that few of us get the advertising right straight away. Even people who have been writing adverts for 30 years can’t always get adverts exactly right first time around. So experimentation is needed.