Maplerock has it right -- the public school system is so much larger and more diverse than the private school system that a comparison like that doesn't have any real meaning. And I don't mean diverse racially, I mean socio-economically, geographically, etc. Our best public school students can certainly compete with our best private school students, and public schools in aggregate are also answering a myriad of other challenges that (most) private schools don't even begin to touch.
The greater point though is that it's good for society to have an educated populace. Vast quantities of research indicate that it's the single best tool we have to use in combating, well, everything: poverty, violence, economic downturns, you name it. I don't think everyone should necessarily go to college, but it also does no one any good to have prohibitive pricing on something that has the capacity to benefit any and everyone. I'm better off if my neighbor and the kid down the street and the kid ten blocks over all have the ability to access the educational opportunities they desire -- be that vocational training, a skill-based associates degree, or a bachelor's in philosophy. End stop. There's zero downside to any of those options. The only question is financing, and that's where the conversation gets complicated -- that's a long and worthwhile conversation to have about what we as a society and community do and don't want to fund.
But this idea that 'if we make it free, it loses value' has no historical or comparative precedent. University education is free in a number of countries, and it isn't devalued. It was free to many after WW2 due to the GI Bill -- which was not initially progressive largesse, but an answer to how we could provide a constructive outlet for the vast quantities of unemployed young men -- and that certainly didn't devalue it. Public education in Georgia didn't become less valuable when they instituted the Hope Scholarship allowing any instate student with a B-average to get free tuition. And so forth. Education isn't valuable exclusively because of the price-tag attached to it, or a specific industry-driven skill-set it provides; it's valuable because the process of becoming educated has inherent value.
There was a point in history when we didn't have free public schools. Then we realized: Oh, it's a good thing for a country and community to have an educated citizenry. Eventually we made it mandatory because we realized it's a REALLY good idea to insist that everyone have a certain level of education. If we now live in a world in which specialized knowledge is how people gain access to a stable economic life, then it's of collective benefit to give as many people as possible access to some piece of that specialized knowledge, which represents opportunity. Again, this doesn't mean everyone's suddenly an artiste. For some people this will be welding, for other people nursing, for other people, yes, philosophy. That's up to the individual. (And philosophy will never be more than 0.01% of that group, let's be honest.)
Also, this idea that any degree other than a hard science degree being worthless has no grounding. Yes, hard science degrees have value. So too do many other degrees including rigorous degrees in the humanities, and even the arts. The issue in higher education today certainly isn't that too many people are majoring in philosophy or women's studies or ceramics and lollygagging around for eight years -- these are straw (wo)men that are often cited in conversations like this. (Degree-seekers in philosophy have been plummeting for years even though it was the foundational discipline for education for millennia. And 'studies' and 'arts' programs are a tiny sliver of a tiny sliver of degrees awarded each year.) The current issue is the rise and explosion of 'professionalized' degree programs run by non-profit and for-profit colleges alike that are low on substance and very expensive. They have few proven returns, saddle students with excessive debt, and provide them with neither a differentiated professional skill nor new analytical skills. They are credentialing mills and they are what's devaluing education in this country.
Those programs are taking what was once the path to economic opportunity in this country, and turning it into an avenue toward grinding, punishing debt. Debt that will eventually balloon and be a (partial) catalyst for another financial crisis. That's what's going on in higher education today. It's not about drunk, lazy frat boys having their college paid for -- no one's rallying for that. It's about the fact that there are fewer and fewer opportunities like those described above to work hard and get college degrees without debt. Are there exceptions? Sure. What Falconsrock describes above is a prime example. The military is a major -- maybe THE major -- avenue toward those opportunities. But we need more of those, not less. The idea that you can just get a job after high school and save up enough money to send yourself to college is a totally antiquated idea in most places. Take a look at this chart of base tuition for college in all states:
http://trends.collegeboard.org/coll...tuition-and-fees-sector-and-state-over-time-1
These are the absolute cheapest options, and, as is quickly clear, your luck is largely dictated by geography. And none of this takes into account rent, food, transportation, books, etc. You quite simply will spend between $30,000 and $45,000 on college tuition even if you go to CC for 2 years. Plus lost wages, plus living expenses. If you don't come from a family that can afford to pay your way, you will be saddled with debt. That debt will limit all kinds of other choices, and our economy. There are only rare exceptions to that rule. That's why there are rallies.