They are stopping lessons for 17 minutes in memory of the 17 people killed at Marjory Stoneman Douglas High School.
Pupils at the school, which was targeted by a former student, hugged each other on the football field.
Protest organisers accuse Congress of failing to tackle gun violence.
The White House revealed a plan this week to deter school shootings which does not include President Donald Trump's repeated calls to raise the age for buying semi-automatic rifles to 21.
Instead, it moves ahead with his controversial proposal to provide firearms training to school employees.
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