Those have been popular since I was a pre-teen and teen, too. (I’m 25, now.) They were always chaperoned and produced cranky kids in the morning. However, ours usually had a faith aspect, with at least mass kicking it off and some kind of faith-related activity during the lock-in.
My parents did not let us attend nor entertain the idea until we were in high school. But, when I did attend, it was always really fun. I went from ages 14-16 or so. My Catholic high school also had lock-ins at a sports club, where we could swim, play indoor volleyball, basketball, soccer, jump on the giant trampoline, etc. There was nothing faith-related about those, except for mass beforehand. The parish-sponsored ones usually had mass, then a pizza dinner and mixer with a DJ. Following that, everyone changed into pajamas and there were movies and games until about 3 or 4am, when everyone was required to climb into their sleeping bags and attempt to sleep.
I will say that these events were rarely scandal-free, and there was always a girl or two who would whip out her “nightie” and prance around for about five seconds before parents were called or she was asked to change into something more appropriate. There was also always this air of excitement about spending the night in one big, huge room with the boys merely feet away. Looking back, I don’t agree with kids running around in their pajamas together unless the dress code is sweats or track pants and a sweatshirt for everyone. Plus, sometimes older girls take off their bras since “hey, it’s pajama time!” and then it just gets sort of…well, inappropriate.
But, my experience was high school lock-ins and not middle school, so there is probably some difference there.
Oh, and usually my dad volunteered to chaperone. He wasn’t big on the idea of adults he didn’t know having access to children in their sleep or in their pajamas, “Catholic-sponsored” or not.