Indeed. I had another look at the last few chapters of the main series and didn't spot anything more concrete than what this series ended with. Which isn't much. I'd even like to say that to make it this ambiguous is to say nothing will ever happen. A student and a teacher is a taboo subject, after all, so the default assumption is that nothing will happen, not that something will happen. If you want to say such a pair will form, then there's no use dancing around it. There's no end to students who have a fleeting, immature crush on a teacher (though 99% of time the teacher would not be aware of it, I reckon), so that's the baseline. Even the main series of this one had a specific case of a teacher turning the student down.
The only thing that would speak for this pair is Nagai's stubborn personality. He was called a delinquent of the school because he didn't care about others' opinions that much. So, he wouldn't stop chasing Hinohara because of any taboo labels. However, if she made it absolutely clear that nothing will happen, even after he grew up, he would stop. Did she? Did she not? Only the author knows but won't tell. This extra chapter revealed she never even went to his concerts. A pretty adamant way of enforcing separation and distance if you ask me, but we still won't know.