It’s fine for multiple people to post their own photos of the same organism - iNat records encounters, and each one of those is a separate encounter. Anyone using iNat data should understand its issues when it comes to abundance.
If two people encounter the same organism and only one gets a photo, letting their companion use it (with permission) is OK from time to time. It’s best to also note that you got permission to use the photo. But people really should be posting their own photos, and using a friend’s photo shouldn’t be a regular thing.
IMO this particular case sounds like something that merits an explanation and a message to the teacher about best practices.