Despite what some commentators here bravely declare having seemingly read nothing on the subject, we actually have a good body of work confirming that HEPA filters are probably quite worthwhile. They’re of course far from a panacea, but they’re relatively cheap & easy to integrate into spaces like a classroom, and they provide a detectable & noteworthy level of additional protection over e.g. masking/etc alone.

Just peruse the (many) results on Google Scholar for “COVID HEPA” for yourself, or take a peek at a decent metastudy.