Here in the African summer, I have to watch out for over-developed negs! My developer can easily pick up 2 degC during a 5 min developing time. Our ambient temperatures in the late afternoon / early evening can be around 30-32 degC, and the cold tap water is around 28. To make a cold water bath is schlep, but it is the only way to stay around 20 or 24.

It is possible that your observation of thin negatives has more to do with under-exposing the film than with under-development. But it could also be that novices don't get the subtle parts of development quite right, e.g. agitation technique, ensuring freshness of the developer (easy to accidentally oxidise a batch of developer), temperature control etc. How far the negatives are off should be an indication of what went wrong. If it is a stop or so, then probably blame the camera light meter. But if it is way off, two or more stops, and inconsistent, then it is probably the result of development difficulty. I learned developing films by watching an experienced guy doing it. It would be great if you could offer those novices a demonstration. It will definitely help them. One thing you could try (at their expense) is to expose two rolls of film identically, then develop one yourself and let them develop the other. I always keep one or two rolls of bulk film in a loader for such exercises, and load 10-12 frames at a time.