Thanks to Utah Veteran for the great picture!
Some of the tiny particles inside the clouds - the cloud seeds - are made of pollution from cars and factories. Because people drive more on weekdays than on weekends, there is more and more pollution in the air as the week goes along, so that pollution rates are highest on Saturdays and then lowest on Mondays after the weekend. In some cities, especially along seacoasts, the added pollution makes it as much as 22% more likely to rain on Saturday than on Monday (check out this link). So if it seems to you that it's always raining on the weekend and sunny when there's school, that's caused by people driving, and you can help fix it by walking or biking to school.
You can find out whether this is really a problem where you live. For a month or two during the fall or spring, record whether it rains or not every day (You might find it interesting to also record whether the weather report predicted rain for that day or not). Is it really more likely to rain on weekends than weekdays?
Bibliography and further reading about rain:
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