Most modern camera manufacturers use a verticaly traveling metal focal
plane shutter in their SLR and DSLR cameras.
It is definitely a symptom of a sticking shutter.
Don't forget that the image is _inverted_ at the film plane and the
bottom of the scene is captured at the top of the film frame.
A dark stripe on along the top of the negative (which is the bottom of
the scene) is a symptom of a sticking shutter, it starts a bit slow
(over-exposing the top edge of the film) and then runs at the correct
speed across the rest of the frame.

Have a good look at any AF Nikon SLR, a Canon EOS SLR, or any of the AF
Pentax SLR camera bodies, they all have a verticaly traveling metal
focal plane shutter.
Even the Voigtlander rangefinder series have a verticaly traveling metal
shutter.
The only "modern" manufacturer to use a cloth focal plane shutter is
Leica and that is only in their rangefinder cameras; Leica SLR cameras
use a verticaly traveling metal shutter.

It could also indicate that the shutter is sticking slightly partway
across the film gate, and over-exposing that area of the film.

Luis Ortega spake thus:

So it's gotta be an exposure issue, and not light leaks. Most likely
uneven exposure due to dragging shutter curtains, as others have
suggested. A trip to the repair shop for those cameras may be in order.

By the way, if the uneven exposure is along the bottom (long) edge of
the frame, the offending camera has vertically-traveling curtains; along
the short edge, horizontally-traveling ones. Might help to sort out
which camera took which pictures.

If this is 35mm, something everyone seems to be assuming,
I'll toss my hat in with the sticky-vertical-shutter crowd.
Vertical shutters were used on Exa's and Exakta 500s -- if there is
one of these hanging around I'd bet money it's the one with the problem.

A light leak is the usual culprit but 35mm cameras as a rule don't
leak light into the mirror box.  Is the film fogged or overexposed?

You should only see this in contact sheets from one camera and
one photographer unless the class shares common camera(s).

Probably a good idea to report the problem to the camera owner/
class instructor.  The problem will only get worse with time and
the shutter may stop completely in cold weather.

 If the effect is inside the frame its obviously happening
in the camera. Since it seems to happen on different edges
in various pictures its not likely the shutter. It _might_
be an internal reflection of some sort. The way to find out
is to put a small piece of ground glass in the film gate,
open the shutter (T exposure) and shine a flashlight intot
the lens from different angles. Actually you may be able to
see a reflection just by eye. The cure is to paint whatever
is causing the reflection with flat black paint or some
other light absorptive material.
  It is possible for a focal plane shutter to cause a band
of under or overexposure at one end of its travel but it
will consistently at the same place on each frame. It is
also possible for a focal plane shutter to cause spots or
steaks on frames if it has holes in it, but this sounds like
something that is not shutter related.
  If the camera is an SLR check for light leaks from the
finder when the mirror is in the up position. Sometimes the
light seals in SLR's get old and don't work. Unless you eye
is pressed right against the finder some light can get in
and cause some form of fogging. I am suggesting this only
because your problem seems to be a little unusual and the
cause may be a bit unusual.

Sounds to me like shutter bounce in a camera with a vertically
traveling shutter such as an FE, FM2, F3 and many others.
Manufacturers such as Nikon switched to vertical travel in an effort
to increase the flash sync speed, top shutter speed and for other
reasons I'm sure.

==
    John S. Douglas
    Photographer & Webmaster
    www.legacy-photo,com
    www.xs750.net