Is this a sign of pregnancy?

Hi,
Breast tenderness is a commonly experienced premenstrual symptom. Irregular periods are also common for a teenage girls who have recently begun to menstruate.
Conception happens when an egg is released from an ovary. This is called ovulation. The egg, which can live for 12 to 24 hours after leaving the ovary, travels down the fallopian tube and waits to be fertilized. When sperm, which is in semen (the fluid ejaculated from the penis), enters the vagina, it travels and penetrates the egg. This is called fertilization.
Yes, your friends are right. Unless, sperm enters your vagina by sexual intercourse, or comes in contact with your genital area, such as your boyfriend fingering you with semen on his fingers and hands, you cannot become pregnant. Basically, sperm needs to fertilize an egg for pregnancy to result.
Try to relax and wait to see if you get your period. I would strongly suggest that if you choose to be sexually active, you must consider if you feel ready and not just acting under pressure. At 15 years old, you are still very young and need to educate yourself about pregnancy, birth control and STDs. There is a wide variety of information on these topics on this website.