This is perfectly legal to do. But just because there is one middle schooler on a high school team doesn’t mean that the team can compete in middle school competitions.
Another side note: If there’s a middle school team that contains AT LEAST one high schooler, then they CANNOT compete in middle school competitions either.
What’s really important is who’s on the drive team, there’s no rules that prevent a high schooler from building or programming a robot, there’s not even rules preventing adults from building or programming a robot (though if adults build or program a robot it completely misses the point of STEM education).
my organization technically has two teams (621A being highscool and 621B being middleschool), however everyone works on, and practices with both robots, but only middle schoolers drive the 621B robot in a competition.