When people say "X should be free," they mean "X should be free at point of use," with it being paid for at some other juncture by some collection of people or entities that can afford it, with some moral judgment being chosen and applied to inform who is expected to pay for it, and when, and why.
All services paid for by tax money fall under this concept, as do some risk-pooling services like health insurance, always with the caveat that the effectiveness of this mechanism runs on its being funded sufficiently to meet the demands placed upon it; underfunded services provide mediocre or insufficient results and often depend on gatekeeping to reduce access, which undermines services further and also siphons money from the service itself.
The question of just what should be provided (how much food? what quality food? what variety? should it be provided as food or as money with which one can only buy food? what restrictions on type or brand? should it be utilized to disperse surpluses? to enforce healthy eating habits? to direct recipients towards foods whose industries or brands pay for that direction?) is much of the province of politics, but there is no functional difference between paying for school lunches to be free and paying for school to be free.
Behind the scenes, this is a game of causes and effects and of investments and considerations: the utilitarian justification for something to be free at point of purchase is that some benefit is derived from the arrangement.
The convenience of being able to travel freely on roads without being charged by road usage per-mile and per-road, and for not having one's freedom curtailed by road structures designed around monitoring and charging for road use (toll roads have specific, limited access and egress points, limiting one's ability to get on and off them and also creating a captive market for whatever small collection of gas and food suppliers are allowed to set up directly on the toll road) makes it economically and socially convenient for roads to be free at point of use and paid for some other way, via gas tax (which corrollates with personal road usage) or income/property tax (which feeds off gains made possible, in great part, by road usage).
Risk-pooling institutions offer the ability to disperse the blows of adverse fortune which could descend on anyone, turning the possibility of a natural disaster or a crash or a cancer diagnosis from financial ruin into a series of monthly payments, and buffers society from the effects of sudden devastating poverty on a not-insignificant section of its population.
Schools are free because an ongoing supply of educated, work-capable adults are a useful economic resource.
School lunches, when provided, might be free because students do better when fed, or because children are healthier when fed, and healthier children take less medical care and children who get better knowledge out of their free public schooling are a better return on that investment.
Or it might be a moral investment under the banner that children are valuable and precious and should be cherished and provided for and not made to suffer too much for their parents' economic deficits, shaping and reaffirming the social contract(s) under which society operates. This also has value.
School lunches and other food programs in particular, as well, help disperse food surpluses that could otherwise lead to a drop in prices, devastating for the industry that produces them. In addition to feeding children, it acts as a price stabilizer, a valuable economic service.
It can be argued, as well, that anyone suffering an inability to pay for the basic necessities of life has been robbed already, by personal economic exploitation such as underpayment, or denial of access to work they can do, or overcharging for those basic necessities (high rents, for example), or some personal disaster having destroyed their ability to work (disability, mental health), and the recipients of that money that should have been theirs should be made responsible for providing to them what it has deprived them of the ability to buy themselves.
Finally, there are the economic effects of free provision to consider. Most notably, when something is available for free, it becomes impossible to use someone's desperation to overcharge them for it. It also removes "I can't afford that" as a downward pressure on the prices of things. Types of demand will adjust. Increases in consumer demand as people spend freely on something else what they were once forced to spend on necessities will create job demand, which will create more opportunities for those in poverty due to unemployment or in the most low-wage and low-status jobs, which will force employers who utilize on low-wage labor to reconsider how valuable those jobs are and adjust wages and business practices accordingly.
Poverty is a problem in the flow of money. It occurs when income is diverted away or cinched off from a population, whose needs are thus unmet but who then also cannot pass money along the chain of economic exchanges that constitutes a functional and healthy economy. Who cannot, due to decreased input, exert a healthy level of demand for needed and wanted services and products, and pay for them; that is, cannot provide output.
In a healthy economy, money flows. Its purpose is to facilitate the exchange of goods and services, each person taking part as consumer and, when healthy, adult, and able, as producer. When people are denied access to their roles as producers (by high unemployment, for example) or undercompensated as same (poverty wages), their role as consumers is circumscribed, and they cannot provide the draw on the economy that is consumer demand, and a portion of the economy stagnates.
When given something free at point of purchase, this important economic role is restored to them, and the people around them have the economic impetus to provide them with goods and services; the money flows through many hands on its rounds.
Free at point of use services perform a vital role in the economy, streamlining commerce, freeing up economic chokepoints, and providing pump and suction to stagnant places in the economic waters.
Where it comes from is a matter of politics, but it can be siphoned off from someplace where the loss is less painful, or taken from someplace considered unnecessarily glutted to no useful benefit, or invested by those who stand to benefit from the larger situation, or recovered from those entities who have stolen value from the benefitting population and exacerbated their poverty in the first place.
But it needn't be slavery, communism, or magic.