As you are passing through your Adolescent age, it is normal to get such erections on the thoughts of sex. It comes under normal sexual behaviour but if you feel you are always occupied with such arousals and you are unable to focus on other activities of life then you should start Yoga Pranayam Dhyan etc. to calm down your thoughts.
Next Steps
Feel free to chat on Practo if you want to know more about your problem.
Health Tips
Avoid chilly and junk foods, Alcohol, Smoking, Late night movies, visiting porn sites.