why doesn't tax revenue rise at the same pace as student population?
Now it's 50%.
That is just a change in participant behavior. No reason tax revenue would go up 5x without some big hikes.
We aren't just taking a 5% increase because people had more kids (which as you said, should be dealt with by natural tax increases)
I think it makes people as a whole make better choices. If we could get 100% of people to go to college (or even 90% or 70%), I don't think we would be worse off as a nation... but obviously it needs to be free or near free, not burdening everyone with 10 billion dollars in debt.
[0] the US spends twice as much per student on average (including all sources of funding, government and tuition). http://nces.ed.gov/programs/coe/indicator_cmd.asp