I would say a guidance counselor should discuss this with the student and see if there are any reasons behind it (such as being bullied, experiences with parents) and if not then perhaps a referral to a chld psychologist might be of help.
There are many types of bipolar. A person whose moods change that quickly could be called rapid cycling which is common (happens with me). As well bipolar doesn't have to be extreme. Cyclothymia is a mild version of bipolar. Speak to your psychiatrist about all this.
A psychiatrist matches up a person with a set criteria in the DSM-4. Its best to give the psychiatrist all the information you can to help them make that diagnosis. Its not always made on the first visit. Mine changed over time.