Casual study in the USA

I have a New Zealand passport. I am married to a Canadian and will apply for a Canadian Residency shortly.

UPDATE: I've since discovered there is at least a one year delay to get a residency in Canada. So the added issue I face is that when I enter the USA, departing to Canada (as a non-resident there) will not fulfill the requirement that I leave the region of USA/Mexico/Canada within 90 days of entering the USA. Looks like I'll have to depart further afield, which is going to be a pain.

We wanted to engage in some informal studies in the USA. The studies are not at a college or institution. It is private studies, and requires us to be in the USA for a period of about 3 weeks four times a year. We can also, optionally, attend 9 day workshops at the end of every interim month.

To clarify, we want to simply make multiple trips to the USA over a 1 year period, as tourists under the VWP. The fact we are studying is ultimately irrelevant, although pertinent to my questions because at the border I will be asked, "What is the purpose of your visit?".

We'll be residing in Canada during the year in question.

So far I have found out that "casual studies" are permitted on a VWP. But I have not found an official definition. Some (non-Government) sites say that casual studies mean the course is under 18 hours per week. But I didn't find any reference to this on a Government site.

I wanted to ask if anyone knows more about this, or can refer to me an official US Gov site with the information I require?

My question does not relate to long-term study (otherwise it would have an easy answer, and not require posting). It is about short-term trips to the USA, and understanding what the legal view is on these trips. Say for a week at the end of every month, over a year. And for three weeks every third or forth month. Is US Border control likely to have an issue with this? Are they likely to think I am coming to the USA to often? Would it be advisable to not disclose that I am "studying" with a teacher there, and to simply say I am visiting a friend (which is still technically true)?