Yes, all medication can loose its efficacy over time as the body becomes used to it. However, from your post it is not clear whether there was a significant improvement at the start of taking Ritalin and then a steady decline, or if there was a sudden decline.
A sudden change in academic performance or behaviour could be as aresult of something other than medication such as a fight with a friend or bullying at school or change in home circumstance.
If the work has gotten more difficult one would assume the majority of the class would begin to struggle to some extent - this would be a question for the teacher.
If you are unsure of the effectiveness of the medication, have an honest discussion with your daughter regarding her slip in marks and see what her thoughts are. Concerns regarding efficacy and dose should always be addressed with the prescribing doctor.
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