Perhaps the simulation needs a Dunce Rate, to reflect that fact that even in have have an over-supply of university places, not everyone is cut out to get a degree. You'll still have people who drop out of school and move into forestry. If a plenitude of schools still left some uneducated people it would be a good driver to keep some low level jobs around instead of just building shiny lawyers offices.
If I were overhauling the education/jobs system of the game I'd do this and another couple of things.
I'd have it that "educated" was the normal level, most immigrants would have this level of education and almost all jobs would require "educated" cims at a minimum. Jobs for uneducated cims would be few and far between. Being "uneducated" would place cims in an underclass that has severe unemployment (and thus crime) problems.
This would punish, severely, not providing enough elementary schools for your cims, something which in the current game is not a major problem and sometimes a legitimate tactic. If kids can't get even an elementary education they're going to be a social problem for the rest of their lives.
Add to this a "failure rate" at each stage of education like Lord Canterbury suggests and you've got a distribution of cims across the skills rage. Plus, even with tons of education buildings, you'd always have a supply of uneducated cims being a problem you have to deal with, which would add an extra challenge to the game [especially in regards to crime which is very easy to deal with in most cities]
I
think all of the above could be done with mods. I have one other suggestion which would probably require a change to the base game.
The failure rate could be reduced by extra funding, policies (i.e. a "smaller class sizes" policy might reduce the failure rate in schools but reduce the capacity per building) and some specialized buildings. The latter could be quite interesting - libraries, museums etc. wouldn't increase capacity of education services but would make them more effective by reducing the dropout rate.