If there is ONE demonstration in the entire physics spectrum which calls for good equipment, it is this one. I remember my teacher telling me (1965) that I should be able to see Newtons Rings when using sodium lamps, or diffraction patterns when using a spectrometer, but it was only the regular "smacks" which persuaded me to agree with him. This is THE classical indicator that light is a wave (as required for Y11 - I do know about wave-particle duality, etc). I have seen so many teachers TELLING pupils (Y8?) that "light is a wave" but not being able to offer any evidence - that is not science IMO. Get onto your head-teacher and say you MUST have a laser but consider Risk Assessments and please don't use it to show "light travels in straight lines".