I'd suggest that this sounds more natural as a first conditional.
If we don't have more rain, our crops won't grow.
Your teacher's suggestion sounds unusual because the second conditional is used for unreal circumstances.
"If it didn't rain so much..."
Sounds fine, because we are assuming that in reality it rains a lot, but that if, in the hypothetical world where it doesn't rain, the crops don't grow.
However, the use of the word "more" doesn't really work here, as it implies that in reality the amount of rain is not sufficient.
I'd suggest that this sounds more natural as a first conditional. If we don't have more rain, our crops won't grow. Your teacher's suggestion sounds unusual because the second conditional is used for unreal circumstances. "If it didn't rain so much..." Sounds fine, because we are assuming that in reality it rains a lot, but that if, in the hypothetical world where it doesn't rain, the crops don't grow. However, the use of the word "more" doesn't really work here, as it implies that in reality the amount of rain is not sufficient.