Just send a email which gets straight to the point. Make sure you mention how much you like your job & how the added education will make you a better employee & that added knowledge could be of value to the company you work for. So by giving you the time off will be a "Investment" to both your & the Employers future as your Employer will be able to benefit from a employee who has a degree & this degree could be utilized down the track if you are promoted.
I'd go with the truth, maybe suggest dropping to part time so he's not left without the needed help.
Do it face to face,or by phone. An email is too impersonal,your Boss may not receive it, it may not answer any questions he may have and 1 or 2 other reasons.