This is one of the most annoying things about grade school science labs for me. We already know going into the lab what the result should be, and if we get something different we know we need to figure out where we went wrong. The fact is that high school students are never going to get the same accuracy and precision as professional experimenters, but labs should still be conducted before the lesson on the principles involved. Then afterward teachers can ask the students whether they think their numbers are too high, or too low, or whatever.