Can an on-arrival visa be converted to a student visa for Assumption University?

i don’t want to assume anything or make any assumptions

In my observation ABAC HR is well known among Bangkok immigration officials and she handles the visa of students and faculty. She could answer you and I hope it could be done

There are people at the university that will do that for you. Go see the administration

The school is the only one who can answer this. Each school knows their local immigration office and the regulations. Because each office and each school is different, the school will know if they're able to provide the documentation needed.

Before Covid and even during Covid, University students have always been able to easily get an ED visa from their home consulate and it's the recommended method of entry. It's very easy for university students to get this at home.