I should add I work ling shifts, and take AP courses-May stress by a large contributor?
heres an outlet from wikipedia on amenorreah it is a possibility, but you need to go to the doctors to find out.
Amenorrhoea (BE), amenorrhea (AmE), or amenorrhœa, is the absence of a menstrual period in a woman of reproductive age. Physiological states of amenorrhoea are seen during pregnancy and lactation (breastfeeding), the latter also forming the basis of a form of contraception known as the lactational amenorrhea method. Outside of the reproductive years there is absence of menses during childhood and after menopause.
Amenorrhoea is a symptom with many potential causes. Primary amenorrhoea (menstruation cycles never starting) may be caused by developmental problems such as the congenital absence of the uterus, or failure of the ovary to receive or maintain egg cells. Also, delay in pubertal development will lead to primary amenorrhoea. It is defined as an absence of secondary sexual characteristics by age 14 with no menarche or normal secondary sexual characteristics but no menarche by 16 years of age. Secondary amenorrhoea (menstruation cycles ceasing) is often caused by hormonal disturbances from the hypothalamus and the pituitary gland or from premature menopause, or intrauterine scar formation. It is defined as the absence of menses for three months in a woman with previously normal menstruation or nine months for women with a history of oligomenorrhea.
 Exercise amenorrhoea
Female athletes or women who perform considerable amounts of exercise on a regular basis are at risk of developing 'athletic' amenorrhoea. It was thought for many years that low body fat levels and exercise related chemicals (such as beta endorphins and catecholamines) disrupt the interplay of the sex hormones estrogen and progesterone. However recent studies have shown that there are no differences in the body composition, or hormonal levels in amenorrheic athletes. Instead, amenorrhea has been shown to be directly attributable to a low energy availability. Many women who exercise at a high level do not take in enough calories to expend on their exercise as well as to maintain their normal menstrual cycles.
A second serious risk factor of amenorrhea is severe bone loss sometimes resulting in osteoporosis and osteopenia. It is the third component of an increasingly common disease known as female athlete triad syndrome. The other two components of this syndrome are osteoporosis and disordered eating. Awareness and intervention can usually prevent this occurrence in most female athletes.
 Drug-induced amenorrhea
Certain medications, particularly contraceptive medications, can induce amenorrhoea in a healthy woman. The lack of menstruation usually begins shortly after beginning the medication and can take up to a year to resume after stopping a medication. Hormonal contraceptives that contain only progestogen like the oral contraceptive Micronor, and especially higher-dose formulations like the injectable Depo Provera commonly induce this side-effect. Extended cycle use of combined hormonal contraceptives also allow suppression of menstruation.
It may just be the stress. If you are a virgin, you shouldn't have to worry about pregnancy. If you are really that concerned, you could try visiting a gynecologist.
I would say it is most likely due to exercise and stress. try giving your self some down time, do some relaxation exercises, yoga or some meditation. If you don't want to go to a doctor you could try the scholl nurse she might have some suggestions or a female teacher that you feel comfortable talking to.
I'm dealing with the same thing. I've had my period 3 times in the past 16 months, before that it was regular. I'm 23 years old and at a healthy weight. I also have no sex drive. I started a new job a few months before this issue arose, and that has been a source of stress. I'm not overly athletic, I'm working out less than I used to. I'm less stressed out than I used to be as well. Anyways, my GP said I don't have my period because I don't have any fat cells wherein estrogen can be stored. That didn't help me so I went to a different doctor and he prescribed me these all natural supplements that are supposed to "bring my hormones back into balance" which should bring my period and my sex drive back. I've been taking them for more than a month and I still haven't gotten it.
I saw my GP and he sent me to the lab to get some bloodwork done. If I were you I'd get to a doctor and ask to have some tests done to figure out what the issue is.