The lowest level is Uneducated. This is the state some (many?) people in buildings are when new zones are built.
Elementary schools provide education for uneducated children. Graduates obtain Educated level.
High Schools provide education for educated teenagers. Graduates obtain Well Educated level.
Universities provide education for well educated young adults. Graduates obtain Highly Educated level.
Some people claim the green roads for education building coverage are very limited that as long as you use education service buildings with proper road access and have capacity, you are good to go. Some even say education service buildings are global, meaning you could have elementary school in the corner of the terrain and children will still get educated in the center of the city (unconfirmed info, be aware).
In my own experience the green roads doesn't mean much, but schools aren't global either. Usually I put a school building to a high density residential area and then monitor its capacity and if, or when, it gets full, then I'll create another school right next to it and start the same process all over until school buildings don't fill up to max capacity anymore. In some of my biggest high density residential areas I've had several elementary and high schools in close proximity, its like a campus suddently formed there heh.
Eligible does not mean that many students will attend the school, it literally means how many people COULD BE going to this school type (elementary school, high school or university). You can have eligible high school 7.5k and still individual high school building stats show "high school students in the city 8.3k" which is kind of odd as these numbers don't match.
Library to be blunt, is a useless building, it has no value, don't bother wasting good residential zoning space using it.
To make the education efficient, you need to provide public transportation to education buildings. Offices and universities are the most demanding buildings for public transport, industry the least demanding.
If you place school buildings into evenly spread area, lets say low and high density residential areas, then the low density area school might have only 5% capacity used while the high density school is maxed out 100% capacity. This clearly suggests that school buildings have a range, or at least effective range for easier access.
Children, teens and young adults might travel very far away for the school, however there is no denying that the separation between capacity filling on low / high density areas is a fact.
University is usually used only once if not that large city, or only few times even on larger city. On city like 45k population you can have university more or less in the centralized location and everyone eligible to university level schooling will travel there.
Universities should be placed on high density residential area, you can easily fill 4500 capacity on a large high density residential area, you might even need a second university building if the area is really big.
In high density residential area for example elementary schools (community school 915 capacity, using realistic population 2 addon) you can easily place as many as FOUR (4) community school (elementary school) buildings side by side and they still get filled to max student capacity.
Overall there clearly seems to be a DISTANCE RANGE with school buildings. Place a building in low density residential area and you only get low percentage of capacity used, do it on high density and boom you max out easily...
Is it good to have same capacity for elementary, high school and university? Who cares, just keep it on the green.
Can seniors deaths cause lack of educated workers issue? Hmm not sure, without lifecycle rebalanced addon you get a death wave, but how many of those are well or highly educated seniors is unknown.
Buildings should be distributed where they are needed, not just put down anywhere to increase capacity overall. NO. Just keep your education info view "status bar" on green and you are good to go, but at least TRY to distribute new education buildings to new high density residential areas where you would think they are most useful.
University says eligible 19k with capacity of 13.5k, however ALL three universities have max less than half occupancy, why is that? Well eligible means just that, people who COULD go to school, it doesn't necessarily mean they want to (maybe they are already working in "well educated" job etc).