How does a one-entry ED visa work if I need to leave Thailand for a month?

Fisrt off all, your University is only providing documents to apply for a visa or extension of stay at the Immigration Office.

You get OR a Non Immigrant ED visa which gives you 90 days of stay OR an

Extension of Stay depending on course length with maximum 1 year, what ever comes first.

If you like to leave during this issued stay, you buy a re-entry permit at the Immigration Office or last moment on some International airports. This re-entry permit is valid until and will give you the same date as mentioned in your passport as permitted until xx/xx/xx