Question if you flying to bangkok thailand and you have your student ed visa do you have to have a round trip ticket or you don't have to

Hi!

I got a Student ED Visa from last August and it ends on the 15th of June. However, I have to been in Bangkok on the 17th. I went to Malaysia to get a Touristic Visa while coming back but for some reasons they didn't give it to me. I first thought about overstaying instead but since I have to come back a couple of days in Thailand in mid-July, I won't take any risk.

So I now think about doing an extension of stay of 7 days. Is it possible with a Student ED Visa? Cuz all the informations I find are about touristic visa. Is it easy to get it? Because technically I could leave on the 15th but I'd rather not miss my meeting on the 17th, but if there is big chances I get rejected I prefer to save my money.

I can provide an Invitation Letter for my meeting, Passeport photocopy, ID pictures, what else?

Thank you!