Is a kidney infection for a 15 year old serious?

Yes. Kidney infections are generally more serious than a bladder infection (uti). In a patient who is very young, there is hi possibility that there is an anatomic abnormality causing the infection. All patients should have an evlautaion to help determine the cause to prevent future recurrence. This generally includes imaging studies and possibly cystoscopy (telescope to look in the bladder).
Yes. Kidney infection is serious for anyone, it can cause loss of kidney function, and can be life threatening too.Your doctor should try and find out the cause and see if you are more susceptible to it and help prevent more infections.
Potentially. Certainly should be worked up by a urologist or nephrologist. Child has probably already had a kidney us, but should also have a voiding cysto-urethrogram (bladder) xray to rule out bladder reflux plus a dmsa nuclear renal scan or ct with contrast. Kidney infection can lead to scarring which later can lead to hypertension. Also future kiney failure if in both kidneys or in a solitary kidney.