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There are many reasons why students may find this challenging. One big reason might be because the curriculum and the learning levels are different from one school to the next. For example, you may move to a school and continue with the same class there. However, what is being considered in the class was already considered in your former school. Another reason might be the different schedules. When you change your schedule, it might be hard to get back into the rhythm. This, in turn, can cause issues. (I know this from experience. I've switched schools four times)
What piece? please show us what you are talking about.