Same experience as OP and others here in discovering this setting (also in middle school). While sitting next to a friend I jokingly typed out "net send * [our third friend's name]" intending it as a "haha wouldn't that be wild. OK let's delete this now" quip, but my friend reached over and hit ENTER, we ran away from the computer like it was radioactive, and later discovered it was sent school district wide. We had used a non-user specific account but cracked under the pressure of being grilled as the obvious suspects for any computer shenanigans at our school and being good friends with friend #3.

The third friend's father was LIVID that our friend's name was besmirched by being placed in the contents of the pop-up, later threatened me over the phone, and put pressure on the school to throw the book at us (fulfilling his promise to "TAKE THIS TO THE MAT!"). My friend and I were suspended for a day and banned from visiting the high school computer club for some weeks.

From the district IT perspective the middle school's punishment somewhat backfired because news of our suspension resulted in many more students across the district learning about the command, greatly increasing the # of incidents and becoming a district-wide. They weren't able to figure out how to turn it off for a year+ after. As a temporary measure they made the printer in the high school library print out the originating computer of every net send message, which often backed up printer and caused the HS librarians (my mom happened to be the head one lol) to have to remove these useless sheets from the printer all day.

0/10 would not recommend glob experimentation