The population splits into five education tiers, from unskilled to institute. Every building demands workers from specific tiers — if no university graduates are available, the factory stays unstaffed and produces nothing. See tier supply/demand in the Population tab.
Long term: build that tier's school (high school, university, institute) — graduates join the pool over the years. Short term: bring in workers of that tier from abroad with a Labor Agreement, or reduce the number of buildings that demand it.
Yes: use "bring workers" in the Labor Market — the other country must have unemployed people in that tier, and your country must be attractive (wages, prosperity, safety). Migration agreements set up a steady flow too; arrivals join your demographics.
Either there are not enough jobs for the population, or the jobs are in the wrong tiers. Build industry that creates employment; unemployment drags down both income and satisfaction. With a graduate surplus, people naturally lose interest in those schools.