There's a lot of non sequiturs here.
First off adding students to the class room may not diminish the quality of education (no, I don't take the word of Union teachers who's primary purpose is to decrease work and increase pay, not improve education for children), second the additional tax dollars would not necessarily have gone to the classrooms (there's a history of this ... especially with bills written with "... or other purposes." in them) and 3rd reducing class size still wouldn't rid the curriculum of all the social engineering and other crap that gets in the way of learning (in fact more money would likely increase these).
Also you're not taking into account the law of unintended consequences. So what if schools have more money when the parents of these kids have LESS (many of which a LOT less when they'd have found themselves unemployed ... 103 was going to increase unemployment)?
Why is it that giving bureaucrats and unions more money is "for the kids" but opposing an increase in unemployed parents and the taking of money from parents is somehow not?
Furthermore, it was going to be a lot more than $20/year for most of us (you're missing a couple zeros).
The children of Colorado are better off without 103.