As long as you come from a country which comes under the Visa Waiver programme you can enter under that whilst a tourist but then need to visit immigration to change to the M1 Visa.
However, you would be a lot better just entering on your M1 visa in the first place as the date will be valid from the date on the I-20. If asked by immigration be completely honest - oh and expect a wait in the immigration second review line - has happened to nearly everyone I know who has visited!
Disclaimer : I cannot verify these facts it is just my advice. Contact the embassy for more information.