Crossing US Border from Canada Information Request?

My friends and I have decided to go on a trip to New York City, with a touring group called Tai Pan Tours. Our group consists of three teenagers aged 17, and one aged 19. We are all Canadian Citizens, with the proper ID that is required to cross the border. We do not possess any criminal record of the sort. Our goal was a 3-day trip to New York. So my question is that if we possess a consent form signed by our parents granting permission to go on this trip, which is planned by Tai Pan Tours (but not supervised), would we be able to pass the border without trouble? I was informed that I may be refused permission to cross due to the fact that we are not without an accompanying guardian of the age 21. But from what I've read under the DHS Customs and Border Security document of 'proposed rulemaking', we would be allowed through with a signed parental consent form, with a copy of the ID of our parents, as well as a signature that states that the 19 year old would be the guardian of our group. Of course, the 19 year old would also sign in agreement to it. It also states that for those purposes, an adult would be considered 19 years of age or older. There is no mention of the age 21 within the document.

So would this be a valid method to cross the border without trouble?
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But do you know if the age that's considered an adult is actually 19? Or is it 21?
To be properly considered a supervising adult or guardian for the trip.

*By the way, I appreciate the help with the subject, the whole rejecting thing is really making me nervous