There have been discussions around this and it really depends on whether or not the school is over the £36M turnover threshold and trading goods and/or services (e.g. gym memberships, school uniforms etc).
As we are not legal experts we have found an article or two from those within the profession that might help:
https://www.stoneking.co.uk/literature/e-bulletins/modern-slavery-act-and-its-implications-education-institutions
There are many schools and school trusts that have decided to produce a modern slavery statement as a matter of best practice, alongside those who are deemed to be trading. You can have a look at them here:
https://tiscreport.org/organisations-with-statements/field_sector/education-2181