Answer:
By pointing out that people under 18 and 21 are already barred from certain rights
Step-by-step explanation:
Justice Black said:"And I repeat that, if the time has come when pupils of state-supported schools, kindergartens, grammar schools, or high schools, can defy and flout orders of school officials to keep their minds on their own schoolwork, it is the beginning of a new revolutionary era of permissiveness in this country fostered by the judiciary. The next logical step, it appears to me, would be to hold unconstitutional laws that bar pupils under 21 or 18 from voting, or from being elected members of the boards of education."