There's actually a gradualist solution that never occurred to me before, and probably wouldn't destroy the Schelling point. It may or may not work, but why not treat voting like driving, and dispense the rights piecemeal?

Say when you enter high school you get the option to vote for school board elections, provided you attend a school board meeting first and read the candidate bios. Then maybe a year later you can vote for mayor if you choose to attend a city council meeting. A year after that, representatives, and then senators, and perhaps each milestone could come with an associated requirement like shadowing an aide or something.

The key to these prerequisites IMO, is that they cannot involve passing any test designed by anyone - they must simply involve experience. Reading something, going somewhere - no one is evaluating you to see if you gained the "right" opinions from that experience.

When they're 18 they get full voting rights. Those people who chose not to go through this "voter training" process also get full voting rights at 18, no questions asked - kind of like how getting a driver's license at 16 is a longer process than getting one at 18 starting from the same driving experience.

This way, only the most motivated teens would get voting rights early, and everyone else would get them guaranteed at 18. There is likely potential for abuse that I may not have considered, but I believe with this system any prejudices or biases introduced in teens would be local, rather than the potentially national-scale abuses possible with standardized voter-testing.