Clean air is as essential as good nutrition. Improving air quality in the classroom improves a multitude of things, mostly the health and wellbeing of both students and teachers.
A CO2 monitor will help to assess areas of poor ventilation. Place the monitor in the classroom away from windows, doors and 1m away from the nearest person. Check the monitor intermittently to check that CO2 levels have not gone above 800ppm. If they have action needs to be taken.
If you are able to open the windows this is a good start, cross ventilation (where windows on opposite sides of the room are open) is generally the most effective form of ventilation if possible. If this has not worked to reduce CO2 or your classroom is now too cold to work in then you will need help from mechanical ventilation.
To assess what kind of unit you need measure the size of the classroom. When you know this and the room occupancy, use this tool by CAS.
Ideally your CR box will be placed in the centre of the room, if this is not possible consider building more than one unit.