Just for the record (for others reading this thread in the future), head teachers often cause confusion over this at open evenings by saying things like "If you want your DC to come to this school you must put it first on your CAF".
Either the head doesn't know what they are talking about, or what they really mean to say is "if this is genuinely your first choice, the only way to be sure of it being allocated it is by putting it first, otherwise if you are eligible for whatever school you
did put first, you will be allocated that one, as the system assumes you have put them in the true order of preference".
Of course, there is the chance that some like to boast about how many first preferences they got, compared to the school up the road, but I couldn't possibly comment

.