Is there anything in particular that you struggle with when sitting the in school exams? A common one is not being used to doing questions at exam pace and so running out of time when doing them 'properly'.
Also, when doing questions at home, are you making sure you're attempting them as you would in an exam. Do not check the solution until you are completely sure you can't do anymore( as in, spend 20-30min thinking about it if needed). It's easy to gaslight/convince yourself you understand something after looking at the solution. When in fact, you wouldn't be able to replicate that to another question.