First things first! You will need a passport that doesn’t expire too soon -- at a minimum it should last for 6 months after your anticipated return date. Apply or renew now if you need to!
Visas and Permits to Stay
Thirty days is the maximum duration of a visa-on-arrival as a US tourist in Thailand, so students and interns staying in Thailand for longer than 30 days will need to apply for a student visa.
Essentially your passport is your international ID and your visa is your ticket into the country. Without a student or intern visa, you will not be allowed to enter Thailand, and you will not be allowed to participate in the program.
For Semester Programs
The Thai student visa process has two parts: First, the visa is a digital document emailed to you that you must print out and carry with your passport. You apply for this before you depart for Thailand, and it is required for you to enter the country. Second, a visa extension and “re-entry permit” is an immigration process you’ll complete within the first month of entry.
Semester in Thailand - Mahidol University International College: