Eastwind
I am in the same situation I am a canadian, but have lived in the US for the last 9 years and no ties to Canada. In your situation , I am guessing that your last visa status was H1-B. which means you declared your intention to stay in the US and there is nothing wrong with that , your request/desire is still wihin the limits of the immigration law, I do not see you having any problem with a request for a student Visa . I think your may only have a problem if the immigration officer you are dealing with does not know the rules and regulations.. and this happens a lot .. you may want to use a busy border crossing which will have more qualified immigration officers to deal with any kind of request.
I am not looking forward to going back to Canada for a student Visa . I do not anticipate any problem and you should not either .