Good question! Yes they can. 13-17 year olds must be from the US or Canada to be eligible and have the proper parental consent forms. The only real limitation on the top prizes is that the Grand Prize winner won't also get a Bonus Prize. So if your student team won the Grand Prize, we would not also award you the Best Skill by High School Students. We would give that to the next highest scoring high school team.
Here's the rules section:
IMPORTANT NOTE ON MULTIPLE PRIZE ELIGIBILITY: An eligible Skill may win a Participation Prize, a Finalist Prize, and the Grand Prize. Each Maker is limited to one Participation Prize, regardless of the number of eligible Skills entered by the Maker. The Grand Prize winner is NOT eligible to win a Bonus Prize. A Finalist Prize winner may also win a Bonus Prize but a Bonus Prize may be awarded to a Skill that is not a Finalist Prize winner. An eligible Skill can only win one of the Bonus Prizes.