Different meaning and form............
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The sentences are equivalent. A native speaker would verbally stress either students or only now in order to convey their meaning. The word order would be chosen so that the stressed word falls at the right moment in their overall rhythm.
In written English, the rhythm can still be present, since good writers typically imagine how their words can be spoken and heard. In a fuller text, the sentences before and after your example sentences would set both the physical rhythm and the “conceptual rhythm”, where the ideas and examples are inteoduced at a certain pace.
If you want to look at your examples in isolation, however, we can only rely on general patterns in English expression.
The first term, either students or only now, serves to isolate that concept relative to other concepts.
Students, for example, in contrast to parents, workers, retirees, restaurant owners or whatever.
Only now, for example, in contrast to earlier, later, or never.
But this is not a very strong pattern, which is why the sentences are equivalent.
The second sentence means that for now the students have only taken the central role in the democracy movement The first sentence means or implies that the student have taken the central role in democracy movement after a period of time or on base of other event connecting it