It is a perfectly good English conditional, and as far as I know it works in AmE, BrE, AuE.
The way that conditionals actually work in English is much more complicated than is taught in ESL classes, and it varies slightly from speaker to speaker. Here is the way conditionals work for me.
Every conditional is either a real conditional or an irrealis conditional. For each of these two types, there is a past form and a present form for both the if clause and the main clause. You can use different forms in the if clause and the main clause if they describe different time periods, although you probably shouldn't mix irrealis and real (there are occasions when we mix them, but they're rare).
Irrealis
+---------------------------------------------------------------------------+
| if clause main clause |
+===========================================================================+
| Past If he had ridden to Nottingham he would have been killed.|
+---------------------------------------------------------------------------+
| Present If he rode to Nottingham he would be killed. |
| Future |
+---------------------------------------------------------------------------+
Real
+---------------------------------------------------------------------------+
| if clause main clause |
+===========================================================================+
| Past If he rode to Nottingham he was killed. |
+---------------------------------------------------------------------------+
| Present If he rides to Nottingham he will be killed. |
| Future |
+---------------------------------------------------------------------------+
Note that the if clause for the present irrealis is the same as for the past real, a possible source of confusion. Native English speakers don't generally get confused by this, because the main clauses are different for irrealis and real conditionals.
An example of a mixed real conditional is:
If he rode to Nottingham, he will be killed.
Here, the if clause is in the past, and the main clause is in the future.
So
if it wasn't important enough to be on the exam, then I did not teach it,
is a perfectly good English conditional; it's a past real conditional. And the real conditional is the best one to use here, because it's describing a real and not a hypothetical situation.
There are still lots of conditionals whose exact form isn't covered by this answer, for example the present real conditional:
if he is riding to Nottingham, he is riding into an ambush.
However, this will help you understand the main classes.
