1. Boys often have trouble focusing in class.
2. Boys often have trouble focusing on class.
3. Boys often have trouble focusing on teaching in class.
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Q1: Which one is grammatical?
Q2: Does #1 mean #3?
#1 is best.
#2 would work if you wrote "their classwork" instead of "class."
#3 is wordy. It's better to use #1.