Dropping a solution in here that I got via Message (summarised), when I asked one of the first who unlocked it (I’ll delete this solution if they make their own):
> Have a population of around 120,000, with over 30,000 eligible for University
> Deactivate School's Out Policy
> Activate Education Boost Policy
> Activate Industry 4.0 Policy
> Delete all generic Universities
> Build a large amount of High Schools (25,000 capacity)
> Let the game idle for 1-2 in-game years
> Once you have over 30,000 students eligible for University, build a Campus with room for 18,000
> Activate all Campus Policies
As soon as you open the Campus you should get 15,000 students right away