Formating a chemical equation - Physical states
Overall science is not affected by whether you write subscript or a superscript or in line. Afterall all notation was invented by humans. When professor quibble over these things my suggestion for them is to read the problem of "How many angels can stand on the point of a pin?" Apparently when Constantinople was falling, allegedly scholars were discussing these problems. Anyway, the lesson is that science is more important, notation comes later.

The International Union of Pure and Applied Chemistry (IUPAC), makes recommendations to systematize notation. Show the book "Quantities, Units and Symbols in Physical Chemistry" Third 
Edition, https://doi.org/10.1039/9781847557889. It is available for free.

There was a similar question What is the standard way to denote physical states in a chemical reaction?

but the reference is old. The above reference is recent but there is no difference. Just for your interest, to see how old that reference is, it looks like type-written text!



They suggest to write the state of aggregation as they call it, just next to your symbol like $\ce{Na(s)}$, not as a subscript.