Was it within the distance markers?

If so, then they should've escorted the students outside the polling distance-markers and been done with it. Now, if a kid was being a smart-ass and deliberately trying to electioneer within the distance markers, then either send him to the principal's office or, if he does it again after the principal lets him go, give him a citation or arrest him for electioneering within the distance marker and let juvenile court sentence him to community service with deferred adjudication. Better yet, make the kid write an essay on the importance of the secret ballot and the reasons why electioneering near the voting booth isn't allowed.

The latter is only for those students who are deliberately pushing buttons over and over again.

If it's a systemic problem, then next election, declare that part of the school off limits to students for the day, or move the polls to a building the students don't use or to another location completely.