My hunch is the person was speaking to how our shifting understanding of childhood and disabilities - which has been shaped by sociology - has changed who has been allowed to go to school, but I'm not confident what they're referring to. So, alas, I can't speak to that. Regarding the "right" to go to school, you may get a different answer on a legal subreddit, and cases in the last 20 years - even the last six months - have changed the nature of that answer. But, from a historical perspective, American children have never had a protected right to an education.

First, some quick context setting. There is no mention of education in any form, much less schooling, in the United States Constitution. Due to courts and lawmakers' interpretation of this, based on the language of the 10th Amendment, education has been a matter left up to the state. Thus, each state developed a system of education in different times and following different paths. Some states established that constitutional right when they were founded - especially those connected to the Northwest Ordinance from 1787 (from Article 2 in the Ordinance: "Religion, morality, and knowledge, being necessary to good government and the happiness of mankind, schools and the means of education shall forever be encouraged"), while others updated their constitution to incorporate education ("The legislature shall provide for the maintenance and support of a system of free common schools, wherein all the children of this state may be educated." Renumbered by the New York State Constitutional Convention of 1938.) Meanwhile, Mississippi codified segregate schools into their constitution and rolled back compulsory education when it looked like Black children were going to be given access to white schools. In effect, they were saying if Black children were going to be able to enter white schools, they would lift their obligation to provide an education to any child, including white children.

One of the tensions related to the idea of the "right" to education in the United States is the relationship between a state's responsibility to a child with regards to an education. This concept, sometimes known as universal education is the flip side of compulosry education: if the state requires chidlren to go school, it must then provide an education. We see this concept in international documents such as the UN's Convention on the Rights of the Child. That said, it wasn't until the Brown versus Board ruling in 1954 that it was made plain that a state's obligation extended to children of color, not just white children. Likewise, it was the passage of The Education for All Handicapped Children Act (later Individuals with Disabilities Act) in 1975 that spelled out the state's obligation to children with disabilities, actual or perceived. Later court cases would offer protection for children without permanent addresses, children who recently arrived in the United States, and those for whom English isn't a first language.

So, to a certain extent, it's fair to say that due to a combination of laws and court cases, American schools must provide a seat for every child who shows up. That, though, doesn't necessarily mean a child has the "right" to an education. The best way to think about this is through homeschooling. I give a fuller history of homeschooling here but in short, a child has no legal outlet if a parent elects to not educate them. Some states require a parent register their child with the district so they can do welfare checks while others make it clear parents have no obligation to do anything. In other words, a child can reach the age of majority without receiving a moment of direct instruction in literacy and there is no one to hold accountable for that gap.

If I didn't get at what you're asking, I'm happy to answer follow up questions!