There is no difference between voters who attended school only because they were forced to, and hypothetical voters who weren’t forced to attend school.
We don’t trust kids with the choice to smoke, drink or have sex. Hell, we don’t trust them to watch certain films. Why should we trust them with the choice of education any more than other choices that can screw up the rest of their life?
Who's trusting kids with anything? Do they not have parents?
The point is that students who are not engaged themselves, and whose parents are not engaged enough to ensure that they go, will no longer make school less valuable for all by their very presence.