Learn how to use the three types of Sketchup Inferences in this Sketchup tutorial. Lock an inference to allow reference to another point in your model. Reference multiple inference points to locate the intersection of the two.
My teacher uses a Noise Meter similar to this one. The Noise Meter is a cool way to remind students the correct noise level that they should be using! You just use the paper clip and move it to the appropriate noise level the students should be using.