It is not always an easy task, Kingworm. Not only should a new paragraph begin 'when the topic changes' (often a hard point to judge), but also paragraphs should simply be not too short or two long-- paragraph breaks are in many ways like extended full stops: a breathing or thinking break, a point to bookmark the reading until tomorrow. This is what I would do here: Tomas scribbled at the desk with force, scratching and screeching, disturbing the quiet classroom atmosphere.
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