' To get in out of the rain ' just means to come indoors when it is raining. However, to be 'so ignorant that someone must tell me to do that instead of remaining outside and getting very wet' is an indication of how very ignorant I am. (that) it's a wonder the board didn't have to come by and tell me to get in out of the rain ' is a comparative clause indicating the level of ignorance in a humorous way.
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