Because there are not enough jobs for educated people?
I had 2 of each type of school (gradeschool highschool college, and 1 academy of sciences) but 60% of my population was still uneducated. Quite frankly. I don't think any highschooled person will work on a farm, and I need people to work on my farms.
Once a tropican gets educated, he will always move up to get a job for his education level, or else, what's the point? There aren't much jobs that require a college education.
Tourism, farming, entertainment buildings
Factories, soldiers, government buildings (except the palace of course...I think...)
Newspaper, radio and TV station for women, academy of sciences for men, SIX government jobs...what else is there?
If you believe in Jesus Christ, have accepted him as lord and savior, and are 100% proud, put this in your sig.
My two cents. Start them young, then they will for sure go to HS because if they already have jobs, they won't quit it just to go to school as it takes awhile before they graduate, unless of course you also decide to pay students 2/3 of their wages through the edict.