Notoriously few young people vote. But the younger they start to vote, the more likely they are to keep on voting.
Here's an idea: Let students at the high school who are over 18 report to school at 10 A.M. on election day, after voting (and if necessary, registering to vote that morning at the polls). An "I Voted" sticker could serve as the needed excuse slip.
I'm sure high school officials could work out satisfactory details for promoting democracy in this way, and also prepare for the resulting favorable publicity.