As a Canadian Citizen, F1 visa to a B1/B2: will I have to leave the US and re-enter?

As a Canadian citizen, you do not normally need a visa at all to enter the US temporarily for many (non-work) purposes. From Information for Canadians from the US embassy in Ottawa:

Canadian citizens do not require a visa to enter the United States directly from Canada for the purposes of visiting or studying.

Do note also that:

A visitor who intends to live, work or study in the U.S. without disclosing this information beforehand may be permanently barred from the U.S.

So you must tell the US immigration officer that you intend to study.

There is also further detail from the Canadian government travel site:

Canadian students are no longer allowed to begin study in the U.S. without a valid Certificate of Eligibility (Form I-20). This document is issued by the U.S. school and sent directly to the applicant. Students must have their Form I-20 in their possession each time they enter the U.S. for presentation to CBP officers. They should also have documentary evidence of sufficient funds to cover travel, tuition and living expenses in the U.S. for at least the first year of schooling, such as a notarized bank statement or letter from a parent/guardian attesting to the funds.

And more info from Student Visa:

Citizens of Canada and Bermuda do not require visas to enter the United States as students, although they must present a valid Form I-20 at the time of admission. For more information see information for Citizens of Canada and Bermuda.

So you would need an I-20 form for your study but you do not need an F-1 visa. You don't need any additional documentation or visa for attending seminars. There is also no need to leave the US between your studies and the seminars.