Because that would be "tôi đang học" The difference between present simple and present continuous.
Try reporting it.
Edit: I posted this 11 hours ago. Now I tried it again and "I learn." is also accepted as correct by Duolingo for this sentence.
Cassie, it usually needs another word to mean school. Although people do often say "đi học" meaning "go to school" or go study
Technically it can mean that. Vietnamese often drop the tense markers (đã), (đang) and (sẽ) in colloquial speech especially when the time frame has been established. For example:
Hôm qua tôi đã đi can just be Hôm qua tôi đi.
Tôi đã đi also carries the nuance of having gone (I have gone).
I believe hoc also means 'school'? At least, that's how I was taught it... :/