How can I make myself study more?

I have straight A’s and I have had them for years so I’m doing fine in school, and I’ve never had trouble studying for tests even if it’s for hours, but lately (basically since the first semester of 9th grade) I’ve just had trouble studying for even 30 minutes and I’ve mostly ended up waiting a day before the test to study for it or even just the very same day of the test. I don’t know how I haven’t failed any of my tests, but I don’t want to risk getting used to barely studying and then having that bring me problems when I’m in 10th, 11th, and so on, especially now that I’m in a school where I can get kicked out for having a C.
How can I make myself study more?
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