Birth control pill question about preventing pregnancy?

I am student teaching with 10th graders currently and my students have been going through a sexual education program that the Health Department presents in two days to all 10th graders. They mentioned something today that got me thinking. They told students it was very important that if they went to the doctor for birth control for the purpose of preventing pregnancy to tell the doctor this and not lie about it and say it was for your period. The presentor said that if the doctors thought it was for your period it was likely that the birth control wouldn't be strong enough to prevent against pregnancy. I have NEVER heard this and thought that any type of birth control would protect against pregnancy if taken correctly, no matter what it was actually prescribed for. Have I been misinformed this whole time?