I find in middle school most kids naturally lose 5-20 lbs depending on their body style. Asking a kid to maintain a weight that they are at or slowly dropping during season is acceptable and teaches them the importance of making weight. Cutting weight is different, but younger kids fluctuate weight a lot. I kind of feel the same way for high school. Hard for kids to learn and train when cutting weight all the time.
Officially, no to plastics. Did I use them in High School, yes. Used responsibly, they are not much different than a sauna.
Rare cases of kidney damage, usually the cases that result in death is complete organ failure typically starting with kidneys.
Not sure if there is a link to kidney damage from long term use. My medical education tells me no, but I guess if you were cutting weight year round than it could.