Look closely at the student numbered 5.
Compared with the others, there appear to be subtle differences around the face, neck, jawline, and overall build. The shoulders also appear somewhat broader, which may be part of the visual clue used by the puzzle creator.
The differences are deliberately subtle.
That is why many people will look at the image several times before noticing anything unusual.
So, is No. 5 the answer?
The intended answer appears to be #5.
