Posting a photo does not require permission of the subject as long as the photo was taken lawfully (i.e. taken with the subject's permission).
Was the photo taken with their permission?
It probably was taken with their permission, unless a reasonable person would have expected privacy in their position.
Some examples of photos taken without the person's permission:
A photo of someone standing in their bedroom in their underwear, taken from the yard outside looking through the window
A photo of someone in a public bathroom, taken with a hidden camera in the bathroom
Some examples of photos taken with the person's permission:
A photo of someone standing on a public street saying "You don't have my permission to photograph me"
A photo of a friend in a friend's house, taken by another friend while at the house, assuming the photographer was not there unlawfully (i.e. was invited)
A photo of another student in class/on campus at school/university, taken by someone lawfully at that place.
The point is whether the person the photo was taken of could reasonably have expected privacy when the photo was taken.
If a reasonable person wouldn't expect privacy in the same situation, then you had their permission to take the photo, and then whether or not it is lawful to post (i.e. publish) the photo without their permission becomes the question.
So is it lawful to post the photo without the subject's permission?
Assuming you took the photo lawfully (i.e. you took the photo with their permission), then you are free to post it (i.e. publish it).
How about posting a photograph without the photographer's permission?
If someone else took the photo and owns the copyright, you would need their permission or to purchase the copyright from them, otherwise you could be liable to them for civil damages, even more so if you could reasonably expect to profit from publishing the photo (including adsense revenue on a blog).
It would be useful to know the circumstances of how the photo was taken and who owns the copyright for your specific example.