Method A is not really a weak check, it's bound to catch tons of cheating students, it's just tedious for staff to perform. Sounds like your friend's school is

Method A is not really a weak check, it's bound to catch tons of cheating students, it's just tedious for staff to perform. Sounds like your friend's school is misusing the tool. Seeing that students are all using the same references is pretty interesting information, as long as you don't leap to conclusions. Likewise, seeing that someone is not citing references properly may not be evidence of plagiarism but nevertheless it's good for students to adhere to reference standards. When I used Turnitin I had to keep below a certain percentge of 'quoted' text. I found this slightly annoying but I could easily avoid it by replacing a quotation with my own summary + a citation. It's just a stylistic choice that is being enforced through the system. Also, no % of plagiarism should be acceptable - beyond the residual/accidental amount you get from coincidental phrasings.