I have a lot of complicated feelings and thoughts about this, but one thing that immediately jumps to my mind: was the IRB (Institutional Review Board) consulted on this experiment? If so, I would love to know more details about the protocol used. If not, then yikes!

Turns out that under the USA Code of Federal Regulations, there's a pretty big exemption to IRB for research on pedagogy:

CFR 46.104 (Exempt Research):

46.104.d.1 "Research, conducted in established or commonly accepted educational settings, that specifically involves normal educational practices that are not likely to adversely impact students' opportunity to learn required educational content or the assessment of educators who provide instruction. This includes most research on regular and special education instructional strategies, and research on the effectiveness of or the comparison among instructional techniques, curricula, or classroom management methods."

https://www.ecfr.gov/current/title-45/subtitle-A/subchapter-...

So while this may have been a dick move by the instructors, it was probably legal.