Defined by Merriam-Webster, a coup, is a "violent overthrow or alteration of an existing government by a small group." Coups are usually large-scale, violent affairs that are nationwide and attack the very centers of governments, often through use of the armed forces.
This twitter user, Karl, is using a case of several school board members of Newberg, Oregon that have resigned due to conservative members taking over a majority of the board. This is true.
Twitter user Karl also seems to be indicating that this instance is part of a larger national pattern of conservatives consistently getting elected to school boards to push their agendas. This is indeed a national pattern as seen by
The New York Times: https://www.nytimes.com/2021/10/27/us/the-conservative-school-board-strategy.html
And Forbes: https://www.forbes.com/sites/petergreene/2022/11/17/right-wing-school-board-members-are-making-their-presence-felt/?sh=3db417a34ea2
However, to the extent to which that this is a "coup" is up for debate. Conservatives have been known to ban certain texts they deem harmful from schools, as well as enforce stricter, more limiting curricula. The extent to which this has caused violence or "threatened democracy" is still yet to be seen. Troubling for some, yes; a violent coup it is not.