This spray is available through theatre suppliers and companies such as Maplin (www.maplin.co.uk) and Terralec (www.terralec.co.uk).
It consists of a large aerosol can which can be used alongside lasers and lights to enhance the beams by producing a 'smoke' effect. The smoke is actually a very fine mist which hangs in the air for a few minutes. It allows laser beams to be seen much more clearly especially in darkened rooms. It is also less messy than the usual chalk dust as well as being far less expensive and more convenient than conventional smoke machines. The spray is intended for use at parties and events but works just as well in the classroom.
The can contains enough to fill several rooms although only a small spray of 5-10 seconds produces enough mist to get a good effect.
The spray in the photo is called 'Magican' There are others available which are essentially the same product.