***Its whats called parasuicide !
An apparent attempt at suicide, in which the aim is not death.
People who would attempt suicide do not discuss it , or talk about it neither do they give any clue to what they maybe contemplating.
What this girl is doing is looking for attention......3-4 cuts should tell you the story !
She needs counselling but as with most of these people once it gets them the kind of attention they are seeking it becomes a habit of sorts and they use it recurrently.
However it is a major risk factor leading to suicide and therefore you should convey to the concerned school authorities what you know.
There are several treatments like psychosocial counselling and if need be dialectical behavioural therapy can be implemented.
Self injury is often not a type of parasuicidal behavior. It's a coping mechanism. However, if the girl has very superficial wounds and is showing them to other people while smiling, I would consider either embarrassment (per the smile, not the showing), or attention seeking behavior. Likewise, the evidence of only 3-4 cuts suggests that she is new to the practice, and having exposed the superficial injury to a peer in a mixed setting would point to her seeking attention.
Self injury when used a coping mechanism (which is most often the case), is a very complex issue. There are a multitude of reasons why an individual (often women), do so, and while this girl may be the exception, rarely is it done for attention or as a feigned suicide attempt.
I would like to suggest though, that while this may be for attention, her actions reflect that she is disturbed. Anyone who seeks to harm themselves for validation have issues that should be addressed by a qualified therapist.
Either way, this should be reported to the teacher/school counselor. They will address the situation from there.