I don't think any school would be happy with you booking extra slots just in case you need them
It was my instructor who suggested double booking! You don't need to cancel any lessons. At present you are only getting half the lessons you want? Right? Book double the number you need and the weather will even it out for you.
You can be quite sure other students are doing it. That's why you can only get a booking a month ahead.
What does make the schools cross is if you cancel after looking at the forecast and (as often happens) the weather turns out flyable after all. Phone the school before you set out from home to find out what the situation is. Don't cancel before then.
They also give priority to students waiting to do cross country. They don't tell you that, they'll give some other excuse. Don't worry - you'll appreciate being given priority when it's your turn!