Update: The gentleman at Immigration today was cheery. I greeted him good morning and he returned my greetings. It's a first time for me and it's a positive start.
He took a bit of time to browse the history in my passport. And he asked how long I will be staying. So I told him I'll be going to study thai at chula.
While he was still going through my passport, I offered to show him the email from chula which I kept on my phone. I think as he was reading it, it might kind of lend some credibility to support my case.
A while later, I heard him conversing with a colleague vaguely, probably about what visa he should give me. But I am not exactly sure what was said. In the end, he passed me my passport with a smile. I thanked him and am now back in Thailand.
Total time at Immigration counter was less than 10 minutes.