MyBB has a collection of text editors you can chose from all you have to do is to make one default and everyine will be forced to use it as it's the only one that will show across the board.
To change a default editor in you case you want to make Sceditor as such got to:
Admin Control Panel >> Templates & Style >> Themes >> Your theme Inside theme settings under Editor Style "Specify the style to be used for the MyCode editor in this theme. Editor styles can be found in the jscripts/editor_themes folder." from drop-down menu select one and then click on Save Theme Settings.