> Another solution is to let you go to school for free and then pay a percent of all your future earnings

In Australia we have a system somewhere between this and a loan.

You go to (heavily subsidized) university, tuition is paid for by the government with an automatic interest free loan.

Once you earn above a particular (somewhat generous) threshold, your employer automatically starts taking a percentage of your income (depends on income) and pays it towards the loan.

There's no interest but the loan amount is indexed against inflation.

I graduated from one of the country's top universities with $24k AUD (~$18k USD) debt to the government and paid it all off early voluntarily. It used to be that voluntary payments got you a 5% bonus but that's gone now.