I know it's been a while since you asked the question - did you ever submit your lab report? What exactly was your teacher looking for? And in what country
I have some limited legal training in the US (paralegal certification, patent agent registration), and I don't know of any meaning for "a report usable in a legal case". If you wanted to use a lab report in a legal case, you would want a lab report that matches the best practices in the industry. In other words, exactly what you would typically prepare for an A+ lab report. The thing that makes it usable in a legal case would be your deposition, testimony, or affidavit, and the chain of custody for the sample. In other words, if you had a forensic case that depended on the amount of calcium carbonate in the unknown water sample, you would run the experiments properly, write up a good lab report, and then take the stand in court or in a deposition to explain all of the chemistry behind your test in a way that a juror with no chemistry background could understand, describe the results you obtained, describe the accuracy that you can usually expect from the test, and then describe how you obtained your samples and how you ensured that there was no contamination of any of the samples. Then the opposing lawyer would try to demonstrate that your procedure was invalid, that your testing method was flawed, or that your sample was either contaminated or couldn't be positively linked to the case. The judge (in a bench trial) or jury (in a jury trial) would then decide whether or not your results were valid and how much weight to assign them in determining the guilt or responsibilities of the parties involved in the trial.