You can build one of these for just under $100, and it will help significantly remove respiratory aerosols that contain SARS-COV-2 from the air. We recommend at least 2 of them in a normal sized classroom. $200.
Keep in mind – something is better than nothing. Even if you did the single filter build, which would be about $36 with the filter AND fan, it would help significantly reduce aerosols in the room.
And you could save up to buy the rest of the filters, later.