Perspective can be an issue but the people at the back don't worry me too much in terms of their height relative to the band. If those people were some way further back than the band, I would expect them to appear much smaller than the band in such a photo. On the other hand, the people at the front seem to be quite close to the band and therefore, if adults, should be much taller than they appear to be in the photo.
The position of the photographer may be a factor - more precisely, the height of the photographer. If the photographer was standing on a piece of ground or something else that made her roughly level with the band, then people between her and the band could seem to be smaller. Even if this were the case, though, I doubt those people would look quite as short as they do.
I still have a feeling that the land is lower just in front of the fence and that, in order to lean on the fence, those two people had to step down into some sort of gulley. That, coupled perhaps with the height of the grass, gives us a false impression.
If, in addition, they were young teenagers, dressed in an adult sort of way but not yet adult height, then I would suggest the seeming discrepancy in heights would be explained.
Where are Holmes and Watson when you need them?
