Can I enter Thailand on a tourist visa and switch to an ED visa later?

If you get the SETV like Tod suggested, while on a tourist visa, it'll also give you some time to set yourself up here (apartment, etc.) before you start studying at your university.

@Antonia *****

Most schools are taking any where from 2 to 6 weeks to get the MOE paperwork you need to use to apply for an ED visa

Dunno where you heard the eVisa system is "taking 3 weeks" as it's certainly not taking that long at all.

The critical piece of the puzzle is how thoroughly you fill out the application, upload the required documentation, etc.

It's not a race, go slow, know what they're asking for, supply the correct upload and it'll get approved pretty fast (many people get approved in just a day or three)

There is no "tourist visa on arrival", but i think you're talking about comin' in on a FREE 30 day stamp then applying for the in country Non-ED visa at the immigration office in Chiang Mai. Doing it that way requires you have a minimum of 21 days left on what ever stamp (visa exempt or real tourist visa) you show up on.

AND

may require a "special processing fee" be paid to facilitate it being issued.

CONTACT your school about it.

I would say IF you're not gonna get the ED paperwork in time, you should probably apply for a SETV <- single entry tourist visa from the eVisa portal before you come here, that will get you stamped in for 60 days and should be plenty of time to get the in country application filed and processed.