1) It's an exam situation. I don't help you out with your maths question in an exam because the whole point is to show how well you do in exams. I'd be more willing to help you if that wasn't the case.

Yes, Glasses abandoned Izuku. It's import to know why he abandoned him. Because it's an exam. Because he doesn't qualify.

Izuku thought She needs help. She helped me before.

He didn't care about the exam and helped her. Glasses thought about this in the context of an exam, with them being rivals. You don't help rivals.

It's different than say, Glasses running to his exam but found some helpless person on the way to the exam room.